When he could no longer play violin himself, Howard taught,

himanshumalhotra1990 Mahmud6 There are a few problems with that interpretation of D.

First, yes, the lack of a comma is a problem. Without the comma, the word "and" clearly links the two adjoining items ("imparting" and "encouraged"). With the comma, we might in theory see "imparting his knowledge to students" as an aside between two parallel past tense verbs. However, this would still be ruined by the use of the pronoun "them." This clearly refers to the word "students" in the previous term. This links the two parts together, showing that the author's intent is for those two parts to work together. If we had any doubt, we might also notice that none of the other answer choices make "taught" and "encouraged" (or any form thereof) parallel. We'd only see a right answer that deviated from all the others if the other version had a clear flaw of meaning or grammar. To clarify the problem, simply take the proposed "modifier" out and see if the sentence still works:

When he could no longer play violin himself, Howard taught and encouraged them to be as successful as he once was.

Nope! We can drop D.