Choose the correct answer. Children are building nesting boxes, ...?

The correct answer is "aren't they". **Explanation:** This is a question tag. The main statement is "Children are building nesting boxes", which is an affirmative statement. Therefore, the question tag must be negative. Also, the verb in the main statement is "are", so the question tag must use "are" as well, but in a negative form. Therefore, the correct answer is "aren't they". So the complete sentence is: Children are building nesting boxes, aren't they?