The incessant noise of the boring machine made it difficult for us to got to sleep at night.

The correct answer is D. constant.

The word “incessant” means continuous or unceasing. In the context of the question, the noise of the

boring machine was continuous and did not stop, making it difficult for the people to sleep.

The other options are incorrect because they do not accurately describe the noise of the boring machine. The noise was not intermittent (stopping and starting), harsh (loud or unpleasant), or soft (quiet).