1. Il n'y a pas de monstre sous le lit. This means that there is no monster under the bed. The other options ( on, behind and in front of) do not make much sense in this sentence.
2. Vous vendez votre maison? The answer is vendez because the vous conjugation of vendre is vendez.
3. Répondez à la question, s'il vous plaît. The sentence contains the phrase s'il vous plaît, meaning please, which indicates that the subject is vous. The vous conjugation of répondre is répondez, which is why the correct answer is répondez.