Answer:
D. Once things are done, there is no need to dwell on them, to worry about them, or to decide who is to blame.
Explanation:
Parallel structure (also called parallelism) is the repetition of phrases (or clauses) that have the same grammatical structure. The sentence that uses parallel structure is sentence D. Here, the infinitive form of verbs is consistently repeated (to dwell, to worry, to decide). In the rest of the sentences, there is always one form that sticks out:
