What took 700,000 artisans and more than 36 years to complete?
answer is the army of terra-cotta warriors and army is singular so "it" correctly refer to the subject back.

Abhijit K wrote: OA: B..however unable to understand the use of "it"

Hi Abhijit, you perhaps mis-read the OA and hence the reason for confusion. OA is actually C for this.

Looking at the core of the sentence, with B, the sentence would be:

...the army of terra-cotta warriors took 700,000 artisans more than 36 years to complete it.

At the very least, it is redundant in this sentence.

In fact, official questions actually test this redundant pronoun issue quite frequently. Am presenting a couple of examples from official sources below:

With option C, #47, OG-13, the sentence would be:

In 1713, Alexander Pope began his translation of the Illiad, a work that had taken seven years to complete and that literary critic Samuel Johnson, Pope's contemporary, pronounced it is the greatest translation in any language.

Among other issues with this option, it is redundant.

With option E, #84, OG Verbal Supplement, the sentence would be:

The computer company's present troubles are a result of techno-logical stagnation, marketing missteps, and managerial blunders that several attempts at revising corporate strategies have failed to correct them.

Again, among other issues with this option, them is redundant.

Do let me know if you need more official examples of this nature, and I can dig them up as well.

Rivaling the pyramids of Egypt or even the ancient cities of the Maya as an achievement, the army of terra-cotta warriors created to protect Qin Shi Huang, China's first emperor, in his afterlife is more than 2,000 years old and took 700,000 artisans more than 36 years to complete them. `

A. took 700,000 artisans more than 36 years to complete them
B. took 700,000 artisans more than 36 years to complete it
C. took 700,000 artisans more than 36 years to complete
D. 700,000 artisans took more than 36 years to complete
E. to complete them took 700,000 artisans more than 36 years

In A and E, them (plural) does not agree with army (singular). Eliminate A and E.

In D, and 700,000 artisans took more than 36 years to complete has its own subject (artisans) and verb (took) and thus is an independent clause, which must be preceded by a comma.
More importantly, D does not convey the intended meaning.
The bridge took 5 years to build means that the BRIDGE was built.
Similarly:
The artisans took more than 36 years to complete implies the ARTISANS were completed.
Not the intended meaning. Eliminate D.

In B, it is not necessary. The army of terra-cotta warriors took more than 36 years to complete is sufficient. (Just as it is sufficient to say that the bridge took 5 years to build). Eliminate B.

The correct answer is C.