Begrunnelsen står i enkeltvedtaket.

Because begrunnelsen is the definite singular form of en begrunnelse.

• en begrunnelse = a justification / a reason / the grounds
• begrunnelsen = the justification / the grounds

In Norwegian, the definite article is often attached to the end of the noun instead of being a separate word like the in English.

Literally, står is the present tense of stå, which often means stand.

But in this sentence, it is being used in a very common extended sense:
stå i can mean be written in, appear in, or be stated in.

So here, Begrunnelsen står i enkeltvedtaket means something like:

• The justification is stated in the individual decision
• The grounds are set out in the decision

This is very natural Norwegian, especially in formal or administrative language.

Both verbs can relate to location, but they are not used in exactly the same way.

• er i = is in
• står i = is written in / appears in / is stated in in many contexts involving text

If you say Begrunnelsen er i enkeltvedtaket, it sounds more like you are only talking about physical or abstract location.
If you say Begrunnelsen står i enkeltvedtaket, it specifically suggests that the justification is written there in the document.

So står is the more idiomatic choice here.

Enkeltvedtaket is the definite singular form of et enkeltvedtak.

This is a compound noun:

• enkelt = individual / single
• vedtak = decision, resolution, formal decision

In legal and administrative Norwegian, enkeltvedtak is a technical term for an individual administrative decision, meaning a decision directed at a specific person or case.

So:

• et enkeltvedtak = an individual administrative decision
• enkeltvedtaket = the individual administrative decision

Because Norwegian very often forms nouns by making compound words, where English would often use two or more separate words.

So Norwegian writes:

• enkeltvedtak

not:

• enkelt vedtak

This is completely normal. English speakers often want to split long Norwegian nouns, but in Norwegian they are usually written as one word.

Because vedtak is a neuter noun:

• et vedtak
• vedtaket

When a compound noun ends in a neuter noun, the whole compound usually takes the same gender:

• et enkeltvedtak
• enkeltvedtaket

Compare that with begrunnelse, which is a common-gender noun:

• en begrunnelse
• begrunnelsen

The structure is:

• Begrunnelsen = subject
• står = verb
• i enkeltvedtaket = prepositional phrase

So the sentence follows normal Norwegian word order:

Subject + Verb + other information

That makes it a very standard main clause.

Not very naturally.

A word-for-word version would be something like:

• The justification stands in the individual decision

That is understandable if you know Norwegian, but it is not natural English.

A more natural English translation would be:

• The justification is stated in the individual decision
• The grounds are given in the decision
• The reasoning appears in the decision

This is a good example of why direct word-for-word translation is often misleading.

Sometimes, but not always.

Begrunnelse often means:

• justification
• grounds
• reasoning
• explanation of why a decision was made

So in formal or legal contexts, reasoning or grounds may fit better than the everyday English word reason.

That depends on context, but in this sentence it is probably more formal than just reason.

Yes, it sounds fairly formal and administrative.

Two things make it feel formal:

• begrunnelsen
• enkeltvedtaket

Especially enkeltvedtak is legal/administrative vocabulary. So this is the kind of sentence you might see in a letter from a public authority, a legal explanation, or an official decision document.

Yes, that is possible, but it gives a slightly different nuance.

• står i = is written/stated in
• finnes i = is found in / can be found in

Both can work, but står i is often more idiomatic when referring to text that is explicitly written in a document.

So står i is probably the most natural choice here.

You could say:

En begrunnelse står i et enkeltvedtak.

But that changes the meaning:

• Begrunnelsen står i enkeltvedtaket = the justification is in the individual decision
• En begrunnelse står i et enkeltvedtak = a justification is in an individual decision

The original sentence is talking about a specific justification and a specific decision, so the definite forms are important.