Het antwoord is nul.

Because antwoord is a het-word in Dutch.

• the answer = het antwoord
• an answer = een antwoord

This is something you simply have to learn with the noun. A useful extra point:

• singular definite: het antwoord
• plural definite: de antwoorden

So even het-words take de in the plural.

Here, het is the definite article, so it means the.

In Dutch, het can be:

• the with singular het-words
• it as a pronoun

In Het antwoord is nul, it is part of the noun phrase het antwoord = the answer.

This is the normal word order for a simple Dutch main clause:

• subject: Het antwoord
• verb: is
• complement: nul

So it follows the same basic pattern as English:

• The answer / is / zero

Dutch main clauses usually put the finite verb in the second position. Here the subject comes first, so the verb comes right after it.

Because nul is being used as a number, not as a countable noun.

In Dutch, when a number is the complement after zijn (to be), you normally do not use an article:

• Het antwoord is nul.
• De score is drie.
• Mijn huisnummer is zeven.

If you said een nul, that would usually mean a zero as a symbol, shape, or sometimes a grade, not simply the number in this sentence.

Usually not if you just mean the numerical answer.

• Het antwoord is nul = the answer is zero
• Het antwoord is een nul = sounds like the answer is a zero, which changes the meaning

Een nul can be used in other contexts, for example:

• a written 0
• a school mark/grade in some systems

So for the plain mathematical or factual answer, nul is the natural choice.

A rough guide:

• het → often sounds weak, like hut with a very short vowel, or even almost ut
• antwoord → roughly AHNT-voort
• is → like iss
• nul → roughly nül with a Dutch u sound

Two useful pronunciation notes:

1. In antwoord, the final written d is pronounced like t at the end of the word.
   So it sounds more like antwoort.

2. The u in nul does not have an exact English equivalent.
   It is closer to the ü in German or the u in French tu.

This is because of a common Dutch pronunciation rule called final devoicing.

At the end of a word, voiced consonants are often pronounced voiceless:

• written d sounds like t
• written b sounds like p
• written z sounds like s

So antwoord is spelled with d, but at the end it is pronounced with a t sound.

You can often hear the underlying d more clearly when another ending follows, for example in:

• antwoorden = answers

For a yes/no question, Dutch usually puts the finite verb first:

• statement: Het antwoord is nul.
• question: Is het antwoord nul?

That is very common in Dutch:

• Hij is thuis. → Is hij thuis?
• De deur is open. → Is de deur open?

They are related, but not the same.

• nul = zero, the number 0
• geen = no, not any

Compare:

• Het antwoord is nul. = the answer is zero
• Ik heb geen geld. = I have no money
• Ik heb nul euro. = I have zero euros

In many situations, geen is the normal way to negate a noun.
Using nul often emphasizes the exact number 0.

So in this sentence, nul is correct because you are giving a numerical answer.

Not in normal standard Dutch.

You normally need the article:

• Het antwoord is nul.

Without het, it sounds incomplete as a regular sentence. You might see something shorter in notes, headlines, or informal labels, but as a full sentence, Het antwoord is nul is the correct form.