. . . . . . . . the room, the door suddenly swung shut and hit him in the face.

The correct answer is: C. As he entered

The other options are:

• A. On entering: This is incorrect because it implies that the door hit him as soon as he started to enter the room. However, the sentence states that the door hit him after he had entered the room.
• B. While entering: This is incorrect because it implies that the door hit him while he was in the process of entering the room. However, the sentence states that the door hit him after he had entered the room.
• D. On having entered: This is incorrect because it implies that the door hit him immediately after he had entered the room. However, the sentence does not specify how long after he had entered the room the door hit him.

The sentence is about the door hitting the man in the face. The man entered the room, and then the door hit him. The only option that describes this sequence of events is “As he entered.”