A. Could not be the answer because the sentence needs correcting.
B. Does, "Have been mainly due to building practices at the fort" make sense? Ah, no!
C. Were seems a bit blah to me and doesn't really sound right in the sentence
D. Makes sense! "Still, the abundance of preserved tablets are mainly due to building practices at the fort". Does this make sense? YES!!!!
Therefore your answer is D. Are
