Learn Hardcore Icelandic: Merktu svarið.

Merktu is the singular imperative of the verb merkja. In other words, it is the form used when giving a command to one person: Mark ...

If you were telling more than one person, you would normally use merkið instead:

Merktu svarið. = command to one person
Merkið svarið. = command to more than one person

Why is there no word for you in the sentence?

In Icelandic, as in English, commands usually leave out the subject. The you is understood.

So:

Merktu svarið. = Mark the answer.
Adding þú would sound marked or emphatic, not neutral.

This is completely normal for the imperative.

What does svarið consist of?

Svarið is made of:

svar = answer
-ið = the definite article for a neuter singular noun

So svarið literally means the answer.

A very important Icelandic feature is that the definite article is often attached to the end of the noun, instead of being a separate word like English the.