If you stay too long in the sun, your skin ____

The correct answer is "burns."

So, the sentence would be: "If you stay too long in the sun, your skin burns."

Explanation:

• This is a zero conditional sentence, which is used to talk about general truths or facts (things that always happen when the condition is met). The structure of a zero conditional is: If + present simple, present simple.
• "Burns" is in the present simple tense, which is the correct form for this type of sentence, as it refers to a general consequence of staying in the sun for too long.

The other options (burn, burnt, burned) are incorrect in this context because they do not follow the zero conditional structure.