Answer: The right answer is the C) It suggests that the speaker finds the sound of digging to be pleasant and wholesome.
Explanation: Just to elaborate a little on the answer, it can be added that, as an adjective, the word "clean" describes the sound that the speaker heard, and he does not suggest that he dislikes getting dirty, so option A should be discarded. In addition, "clean" has a positive connotation—it can mean healthy, pure, fair, complete, skillful, clear, and smooth, for instance—so it does not suggest that there are differences between the father and the son, or that the son has many chores to do. The right option is, therefore, the C. The speaker seems to find the sound that his father makes when he sinks the spade into gravelly sound as a crystal-clear and healthy sound.
