The sentence talks about the monument that was made thousands of years ago. It is clearly a sentence in the past tense.
So, the correct answer is A) was built.
Option B) were built is incorrect because 'were' is plural while there is only one monument mentioned in the sentence. Option C) is building is in the present continuous tense and so it is incorrect in the given context. Option D) is being built is in the present perfect continuous tense and so is not the correct answer.