The terminus of his progress would be reached when he accomplishes that honour.

The correct answer is D. result.

The word “terminus” means “the end or final point of a journey, course of action, or period of time.” In this sentence, the terminus of his progress is the point at which he accomplishes the honor. This is the result of his efforts, and it is the point at which his progress will come to an end.

The other options

are incorrect. Option A, beginning, is incorrect because the terminus is the end of his progress, not the beginning. Option B, highest, is incorrect because the terminus is not necessarily the highest point of his progress. Option C, terminal, is incorrect because the terminus is not necessarily a terminal point, such as a death or a disaster.