In each question in the following, four alternative sentences are given. Choose the CORRECT one and fill the circle corresponding to that letter in the answer sheet.

Explanation

This option is also grammatically correct. It uses "a" before "knock" to indicate any knock and "the" before "door" to specify a specific door.Option D is grammatically correct and specifies any knock on a specific doorTherefore, option A and option D are both correct, but option D is slightly more specific:I heard a knock on the door.Both options convey similar meanings, but option D specifies a particular door, which is often more natural in context.