Question

Which sentence uses parallel structure?

Answer 1

The correct answer is  A.Once things are done, there is no need to dwell on them, to worry about them, or to decide who is to blame. 
Parallel structure means that the same form of a word is used throughout a sentence - here, the infinitive form of verbs is used: to dwell, to worry, to decide. 
The rest  of the sentences change the form of a word and are not consistent, which is why they don't have parallel structures. 

Answer 2

Which sentence uses parallel structure?

 A.Once things are done, there is no need to dwell on them, to worry about them, or to decide who is to blame.