The correct answer is D) to highlight contrasts to opposing ideas.
One reason that Steinbeck uses parallelism in "The Grapes of Wrath" is to highlight contrasts to opposing ideas.
Steinbeck likes to contrast the differences that separate the rich and the poor, using references to his own youth.
Parallelism is a grammatical term to arrange words of the same syntactical construction in phrases or clauses. It is used to have a parallel structure or the same structure.
The other options of the question were A) to relax the reader with repetition, B) to detract from disturbing images, and C) to soften the intensity of controversial matters.