The correct answer is: C. has begun
The sentence is in the past progressive tense, which is used to describe an action that was in progress at a specific time in the past. In this case, the action is “clipping the hedge.” The past progressive tense is formed with the past tense of the verb “to be” (was or were) and the present participle of the main verb (clipping).
Option A, “begins,” is in the simple present tense. The simple present tense is used to describe actions that happen regularly or repeatedly. In this case, it would imply that Mr. Porter is always clipping the hedge, which is not the case.
Option B, “had come,” is in the past perfect tense. The past perfect tense is used to describe an action that was completed before another action in the past. In this case, it would imply that Mr. Porter had already finished clipping the hedge before his son came around the corner, which is not the case.
Option D, “is,” is in the simple present tense. The simple present tense is used to describe actions that happen regularly or repeatedly. In this case, it would imply that Mr. Porter is currently clipping the hedge, which is not the case.
Option E, “comes,” is in the simple present tense. The simple present tense is used to describe actions that happen regularly or repeatedly. In this case, it would imply that Mr. Porter’s son is always coming around the corner of the garden, which is not the case.
Therefore, the correct answer is: C. has begun