His knowledge of the subject is quite extensive.

The correct answer is D. immense.

The word “immense” means “very great in size, extent, or amount.” In this case, it is being used to describe the man’s knowledge of the subject. This means that he knows a lot about the subject, and his knowledge is very deep and wide-ranging.

The other options are incorrect because they do not accurately describe the man’s knowledge. Option A, “ordinary,” means “not special or unusual.” This does not accurately describe the man’s knowledge, because it

is clear that his knowledge is special and unusual. Option B, “little,” means “not much.” This does not accurately describe the man’s knowledge, because it is clear that he knows a lot about the subject. Option C, “limited,” means “restricted or confined.” This does not accurately describe the man’s knowledge, because it is clear that his knowledge is not restricted or confined.