Это наиболее удобный вариант.

In Russian, the present-tense form of to be is usually omitted.

So:

• Это наиболее удобный вариант.
• literally: This/It — the most convenient option

Russian does not normally say a separate word for is here.

Compare:

• Это хороший вариант. = This is a good option.
• Он студент. = He is a student.

But in the past or future, Russian does use forms of быть:

• Это был удобный вариант. = This was a convenient option.
• Это будет удобный вариант. = This will be a convenient option.

Это usually means this or it in sentences like this.

In Это наиболее удобный вариант, it points to something being identified or evaluated:

• Это... = This is... / It is...

A useful thing to know: это does not change form for gender or number in this kind of sentence.

For example:

• Это хороший план. = This is a good plan.
• Это хорошая идея. = This is a good idea.
• Это хорошие варианты. = These are good options.

So even when the noun after it changes, это stays это.

Because this is a basic identifying sentence: This is the most convenient option.

In Russian, in simple present-tense sentences like this, the noun that names what something is often appears in the nominative:

• Это вариант.
• Это проблема.
• Это решение.

So вариант is nominative singular masculine.

Learners sometimes expect another case because Russian can use different patterns in other structures, but here nominative is the normal choice.

Because удобный is an adjective modifying вариант, and it must agree with it.

Вариант is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives:

• удобный вариант = a convenient option

Compare the agreement:

• удобный вариант — masculine
• удобная идея — feminine
• удобное решение — neuter
• удобные варианты — plural

Наиболее means most in a fairly formal, neutral way. It forms a superlative meaning:

• наиболее удобный = most convenient

A very common alternative is:

• самый удобный

Both can mean the most convenient, but there is a slight difference in tone:

• самый удобный = more everyday, very common in speech
• наиболее удобный = a bit more formal, analytical, or written

So these are both natural:

• Это наиболее удобный вариант.
• Это самый удобный вариант.

The version with наиболее may sound a little more polished or objective.

Yes, absolutely.

• Это наиболее удобный вариант.
• Это самый удобный вариант.

Both are correct and natural.

A rough distinction:

• самый удобный = everyday, conversational
• наиболее удобный = slightly more formal or written

In ordinary speech, many people would probably use самый удобный more often.

Yes, quite closely:

• наиболее = most
• удобный = convenient / comfortable / handy, depending on context
• вариант = option / variant

So наиболее удобный вариант is literally the most convenient option.

A key nuance is that удобный often means something like:

• convenient
• practical
• suitable in terms of ease or comfort

It does not necessarily mean best in every sense. Something can be the most convenient without being the cheapest, fastest, or highest quality.

Yes. Russian word order is flexible, though different orders can change emphasis.

The neutral order here is:

• Это наиболее удобный вариант.

You could also say:

• Наиболее удобный вариант — это ...

That structure is often used when you want to emphasize the most convenient option first.

For example:

• Наиболее удобный вариант — это поезд.
  = The most convenient option is the train.

But for the exact sentence you gave, Это наиболее удобный вариант is the most straightforward neutral version.

The stress is:

• Э́то
• наибо́лее
• удо́бный
• вариа́нт

So the whole sentence is pronounced roughly as:

• Э́та наибо́лее удо́бный вариа́нт

A few notes:

• In fast speech, unstressed о often sounds closer to a.
• That is why это may sound a bit like эта, even though it is still spelled это.
• The written form does not change.

Вариант can mean:

• option
• variant
• possible version

In this sentence, option is the most natural translation.

For example:

• Это хороший вариант. = This is a good option.
• Есть другой вариант? = Is there another option?
• Вариант ответа = answer choice / answer option

So the word is broad, but in everyday situations it very often means option.

Yes, but the meaning would change.

For example:

• Это наиболее удобный вариант. = the most convenient option
• Это наилучший вариант. = the best option
• Это наиболее подходящий вариант. = the most suitable option
• Это наиболее выгодный вариант. = the most advantageous/profitable option

So удобный is specifically about convenience, ease, practicality, or comfort. It is not a general word for best.