Parallel structure is achieved by
using the same number of words in each sentence element
never using infinitive phrases
using grammatically similar constructions for ideas of similar value

The best way is sentence structure throughout. This is important to keep the essay on the main idea. Sentence structures could be simple (one independent clause), compound (two independent clause with coordinating conjunction), complex (a subordinate & independent clause) and compound-complex sentences (subordinate & two independent clause). These include clauses, conjunctions, coherence and balance and even to the number of words you use in your subject and predicate. You must also see to it that when you do parallelism, your sentences still makes sense.

I’d go with letter d