To determine the correct answer, let's analyze the sentence structure and the context provided.

The sentence is: "He was by far the most knowledgeable person ________ on the subject, so it would have been foolish ________ his opinions."

1. **First Blank:**
- The phrase "the most knowledgeable person" suggests that the blank should be filled with a form of the verb that describes someone who is actively or passively involved in the subject.
- "to comment" (A) does not fit well because it suggests an action that is yet to be taken.
- "having commented" (C) implies that the action has already been completed, which doesn't fit the context of being knowledgeable.
- "commenting" (B) fits well because it describes someone who is actively engaged in the subject.
- "commented" (D) is past tense and does not fit the context of describing someone currently knowledgeable.

2. **Second Blank:**
- The phrase "so it would have been foolish" suggests that the action of disregarding his opinions should be in the infinitive form to match the conditional structure.
- "disregarded" (A) is past tense and does not fit the conditional structure.
- "disregarding" (C) is a present participle and does not fit the conditional structure.
- "to disregard" (B) fits well because it matches the conditional structure.
- "having disregarded" (D) is past perfect and does not fit the conditional structure.

Combining these analyses, the correct answer is:

(B) commenting - to disregard

So the complete sentence is: "He was by far the most knowledgeable person commenting on the subject, so it would have been foolish to disregard his opinions."