Rivaling the pyramids of Egypt or even the ancient cities of the Maya

Choice B is actually fine when it comes to parallel structure, but the meaning within the parallel structure makes no sense. As Ron explained above, the sentence would then mean "the army of terra cotta warriors... took 700,000 artisans more than 36 years to complete the army of terra cotta warriors." This means the army of terra cotta warriors created itself, and needed 700,000 artisans and 36+ years to do.

Regarding D, the reason parallelism is broken is because we actually drop parallelism entirely. At the beginning of the underline, after the conjunction, we introduce a new CLAUSE (a subject-verb pair). In this case we have "700,000 artisans took 36+ years to complete." One problem is that this clause doesn't specify what was completed. Now we NEED the 'it.' The other issue is that, since we're starting a new clause, there should be a comma before the 'and' in the non-underline (I almost never use the absence or presence of a comma exclusively to eliminate an answer choice, but most of the time when a conjunction is joining clauses, you should use a comma).

D's structure ends up being: "The army of terra cotta warriors is more than 2,000 years old and 700,000 artisans took 36+ years to complete." The more-correct version of this would be something like: "The army of terra cotta warriors is more than 2,000 years old, and 700,000 artisans took 36+ years to complete it."