This is a good question and let me explain the answer:

Firstly, "The British sociologist and activist Barbara Wootton once noted..." When you start the sentence saying person X noted, then the underlined portion has to start with "that", because "that" correctly addresses what the person noted.

On the basis of that, D and E get eliminated.

C has "and it earned" but the "and" is incorrect because it separates the clauses giving rides to children from "earning annually..." but what we want is a connection, not a separation. So, the and it earned part is not correct.

B uses had been earning but the sentence works well using a simple past tense just like the other parts of the sentence. Also, the comma separating the "giving rides to the children" tends to imply, that the elephant while giving rides, is also doing something else to earn the money which is logically incorrect.

So, A is the correct sentence and this is absolutely fine.

Correct Answer A!