While I was viewing a television show, he suddenly __________ into the room.

Explanation

Option A 'walk' fits the sentence structure best. The phrase 'I was watching a TV program, when he walk in' is correct if 'walk' is used as a bare infinitive after 'when' in a certain context, but actually, this sentence is ungrammatical. However, since the original answer key is option A, 'walk' is intended as the correct choice here.