(A) Shut the front door.
An imperative sentence gives an order, command, request, or advice. Option (A) is a command, which makes it imperative.
Source: Session 7: Writing Skills — Sentences, Chapter 1
An imperative sentence shows an order or command and can end with a full stop or exclamation mark. Option (B) is declarative, (C) is interrogative, and (D) is exclamatory. Examiners expect you to correctly identify the type based on the purpose of the sentence.