riidaa ni erabareta koto ha meiyo desu.

Here, に marks the resulting role or position someone was chosen for.

So:

• リーダーに選ばれた = was chosen as leader

This is a common pattern:

• 部長に選ばれる = to be chosen as department head
• 代表に選ばれる = to be chosen as representative

In English we often say chosen as ..., but in Japanese that as idea is often expressed with に.

選ばれた is the passive past form of 選ぶ.

• 選ぶ = to choose
• 選ばれる = to be chosen
• 選ばれた = was chosen

This sentence focuses on the person who received the action, not the people who did the choosing.

So:

• 選んだ = chose
• 選ばれた = was chosen

Because the meaning is about being selected as leader, the passive is the natural choice.

Japanese often leaves out information that is either obvious, unknown, or not important.

In this sentence, the people who did the choosing are not mentioned because the main point is:

• the fact of being chosen as leader
• and that this is an honor

If you wanted to state who chose them, you could add it:

• みんなにリーダーに選ばれたことは名誉です。 = Being chosen as leader by everyone is an honor.

But leaving it out sounds very natural.

Here, こと turns the whole preceding clause into a noun-like idea.

• リーダーに選ばれた = was chosen as leader
• リーダーに選ばれたこと = the fact of having been chosen as leader / being chosen as leader

This use of こと is called nominalization. It lets a whole action or event act like a noun, so it can become the topic of the sentence.

You can think of it roughly as:

• that one was chosen as leader
• the fact of being chosen as leader

は marks the topic of the sentence.

So:

• リーダーに選ばれたことは名誉です。 = As for being chosen as leader, it is an honor.

The sentence is presenting being chosen as leader as the thing being talked about.

If you used が, the nuance would change. が would focus more directly on that item as the subject, while は gives a more general topic-comment structure.

In a sentence like this, は is very natural because the speaker is making a statement about that experience.

Sometimes の and こと can both nominalize a clause, but こと is usually better here.

• リーダーに選ばれたこと sounds natural and standard.
• リーダーに選ばれたの is possible in some contexts, but it feels less suitable here as a formal, abstract fact/event.

A helpful rule of thumb:

• こと often sounds more abstract, formal, or factual
• の often sounds more immediate, concrete, or explanatory

Since this sentence is making a formal statement about the fact of being chosen, こと fits well.

No important word is missing. 名誉です is a normal noun-predicate pattern.

• 名誉 = honor
• です = is

So:

• 名誉です = is an honor

Japanese often uses a noun directly before です:

• 学生です = is a student
• 問題です = is a problem
• 名誉です = is an honor

You do not need a word like an or the, because Japanese does not use articles the way English does.

They are similar, but not identical.

• 名誉 = honor; something honorable, a distinction, prestige
• 光栄 = honor; often used for feeling personally honored

In many situations, 光栄です is very common when saying I’m honored.

For example:

• リーダーに選ばれて光栄です。 = I’m honored to have been chosen as leader.

Your sentence with 名誉です sounds a bit more like:

• being chosen as leader is an honorable distinction
• it is an honor in status or significance

So both relate to honor, but the nuance is a little different.

That version is possible in conversation, but it is a different structure and nuance.

• リーダーに選ばれたことは名誉です。 focuses on the fact/event itself
• リーダーに選ばれて光栄です / 名誉です focuses more directly on the speaker’s reaction

Also, 選ばれて光栄です sounds more natural than 選ばれて名誉です in many cases.

So the original sentence is slightly more formal and abstract because it uses こと to make the event into the topic.

It is polite and fairly neutral-formal.

Why:

• 選ばれた is plain past form inside the clause
• です at the end makes the whole sentence polite

This is very common in Japanese: a clause in plain form before こと, の, とき, etc., and then a polite ending at the end of the sentence.

Compare:

• リーダーに選ばれたことは名誉だ。 = casual/plain
• リーダーに選ばれたことは名誉です。 = polite

Because when a verb modifies a noun-like element such as こと, it normally appears in its plain form.

So:

• 選ばれたこと = the fact that someone was chosen

You would not usually say:

• 選ばれましたこと in normal modern Japanese

The politeness is carried by the final です, not by the verb inside the modifying clause.

This is an important pattern in Japanese grammar: a verb before a noun is usually plain.

The structure is:

• [リーダーに選ばれたこと] は [名誉] です。

Breaking it down:

• リーダーに = as leader
• 選ばれた = was chosen
• こと = the fact of ...
• は = topic marker
• 名誉 = honor
• です = is

So the grammar is basically:

• [the fact of being chosen as leader] is [an honor]

This kind of structure is very common in Japanese: [clause + こと] は [noun] です.

Yes. リーダー is a loanword from English, and it is very common, but Japanese also has native or Sino-Japanese alternatives depending on context.

For example:

• 代表に選ばれたことは名誉です。 = Being chosen as representative is an honor.
• 班長に選ばれたことは名誉です。 = Being chosen as group leader is an honor.
• 委員長に選ばれたことは名誉です。 = Being chosen as committee chair is an honor.

So リーダー is natural, but the best word depends on the situation.