Final answer:
Option A correctly uses parallel structure, involving tasks Daria engages in to study, both formatted with gerunds for consistency. Hence answer is a. daria decided to study by memorizing the definition to sing a song of the same concept.
Step-by-step explanation:
The question involves identifying which sentence correctly uses parallel structure. Parallel structure, or parallelism, is a balance within one or more sentences of similar phrases or clauses that have the same grammatical structure.
The correct sentence that uses parallel structure is Option A: "Daria decided to study by memorizing the definitions, to sing a song of the same concept."
This sentence maintains parallel structure by using gerunds for both activities--memorizing and singing. In contrast, the other options provided do not display the correct use of parallel structure.