The labourers were digging a canal.

The correct answer is C. A canal was being dug by the labourers.

The sentence “The labourers were digging a canal” is in the past progressive tense. This tense is used to describe an action that was in progress at a specific time in the past. In this case, the labourers were digging a canal at some point in the past.

The other options are incorrect because they do not accurately describe the action that is being described in the sentence. Option A, “A canal is being digged by the labourers,” is in the present progressive tense. This tense is used to describe an action that is in progress at the present time. However, the sentence is about an action that happened in the past. Option B, “A canal was dug by the labourers,” is in the simple past tense. This tense is used to describe an action that happened at a specific time in the past. However, the