• Split #1 - PAST PERFECT?

If two events happen in the past and one precedes the other, we use past perfect (had verbED) for the earlier event and simple past or a time marker for the later event. The simple past tense, a time marker, or some other fact that clearly marks off the later event must be present.

No simple past tense event or time marker exists in the non-underlined part.

Options B and E incorrectly use past perfect. Eliminate B and E.

• Split #2 - IDIOM X was built TO STOP Y

I am not sure how so many people knew this idiom. I did not.
I knew that "for stopping" sounded weird and that I would never have used it.
Yes, we do need an infinitive of purpose, i.e. to stop.
"Built for stopping" is incorrect.

Options A and B use built for stopping rather than built to stop. Eliminate A. (B is already gone)

• Split #3 - present perfect is wrong

Option C uses present perfect "has been built." Wrong verb tense.

Present perfect, passive construction: HAS/HAVE + been + past participle (verbED)

We use present perfect as "a bridge from the past to the present," to express
-- action that occurred at an indefinite time (They have lived in that house for 20 years.)
-- action that occurred in the past and continues into the future (They have become frustrated in the last few hours.)

Option C also has subject/verb error. Dikes . . . HAS been built. Wrong.
Wrong verb, but also wrong because Dikes are plural and HAS is singular.

The answer is D