the army of terra—cotta warriors (army is singular since its a collective noun; here we are referring to army as an entity or as a group)

1) is more than 2,000 years old

and

2) took 700,000 artisans more than 36 years to complete


them is incorrect since army is singular Rule out A
E-> As awkward and clumsy as it can get

B. took 700,000 artisans more than 36 years to complete it

Whats does it refer to army or afterlife?? Ambigous


C. took 700,000 artisans more than 36 years to complete
D. 700,000 artisans took more than 36 years to complete


Between C and D I would go with C is more and took sounds more logical than is more and 700 000 artisans(looks like w eare talking about something totally different)

Regards,
CR