Ответ вполне понятен.

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

So Ответ вполне понятен literally looks like Answer quite understandable, but in natural English it means The answer is quite understandable / perfectly clear.

If you wanted past or future, Russian would normally use a form of быть:

• Ответ был вполне понятен. = The answer was quite understandable.
• Ответ будет вполне понятен. = The answer will be quite understandable.

Понятен is the short form of the adjective понятный.

In Russian, short-form adjectives are commonly used when the adjective is the main predicate of the sentence, especially to describe a state, quality, or evaluation:

• Ответ понятен. = The answer is clear / understandable.

The full form понятный is more commonly used before a noun:

• понятный ответ = a clear answer

You can sometimes use a full form in predicate position, but here понятен is the normal and most natural choice.

Вполне often means something like:

• quite
• perfectly
• fully
• entirely

The exact translation depends on context.

In Ответ вполне понятен, it strengthens понятен and suggests that the answer is clear enough, entirely understandable, or perfectly understandable.

So depending on context, you might translate it as:

• The answer is quite understandable.
• The answer is perfectly clear.
• The answer is fully understandable.

It does not always mean a literal 100% fully in every context; often it simply adds emphasis.

Because it agrees with ответ, which is masculine singular.

The short forms of понятный are:

• masculine: понятен
• feminine: понятна
• neuter: понятно
• plural: понятны

So:

• Ответ понятен. = The answer is clear.
• Фраза понятна. = The phrase is clear.
• Объяснение понятно. = The explanation is clear.
• Ответы понятны. = The answers are clear.

Because ответ is the subject of the sentence.

The sentence is structured like this:

• Ответ = subject
• вполне понятен = predicate

So ответ stays in the nominative, and понятен agrees with it.

A learner might expect another case if thinking of the verb understand, but this sentence is not saying someone understands the answer. It is saying the answer itself is understandable / clear.

Compare:

• Я понимаю ответ. = I understand the answer.
  Here ответ is the object, so it is in the accusative form, which for this noun looks the same as the nominative.

Not here.

Понятен means clear, understandable, or comprehensible.

It describes the answer as something that can be understood.

If you want understood in the sense of someone has understood it, Russian usually expresses that differently, for example:

• Я понял ответ. = I understood the answer.
• Ответ был понят. = The answer was understood.

So понятен is an adjective, not the same as saying that the answer has already been understood by someone.

Yes, Russian word order is flexible, but different orders sound different in emphasis.

The most neutral order is:

• Ответ вполне понятен.

You might also hear:

• Вполне понятен ответ.

This puts more focus on вполне понятен and sounds more marked or literary.

For a learner, Ответ вполне понятен is the safest and most natural version to use.

Yes.

• Ответ понятен. = The answer is clear.
• Ответ вполне понятен. = The answer is quite / perfectly clear.

Adding вполне makes the statement a little stronger or more reassuring. Without it, the sentence is still completely correct and natural.

It is mainly used for things like:

• ответ = answer
• текст = text
• вопрос = question
• объяснение = explanation

For example:

• Текст понятен. = The text is understandable.
• Вопрос понятен. = The question is clear.

With people, Russian usually expresses the idea differently. If you mean I understand this person, you would normally say something like:

• Я понимаю этого человека.

So in this sentence, понятен is describing the answer as understandable, not a person.

The stress is:

• отвЕт
• вполне́
• понЯтен

So the full sentence is pronounced approximately:

att-VYET fpal-NYE pa-NYA-ten

The unstressed о in Russian is reduced in pronunciation, so it does not sound like a strong English o. The stressed syllables are the most important part to get right.