Hi, there. I'm happy to help with this!

First of all, answer choice A is indeed correct, and the comparison is complete. When we are comparing two different individuals, we have to identify both --- "A is greater than B". Here, the comparison is the same individual (here, the gyrfalcon) at two different times -- (a) now, vs. (b) some time in the past when DDT was used.

To be hyper-explicit, one could say:

The gyrfalcon, an Arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater than what its numbers were when the use of DDT was sharply restricted in the early 1970's.

but that is usually considered too wordy, so the red words are dropped --- they are implied by the clause beginning with "when".

Choice A is perfectly grammatically correct, with a complete comparison, so it's correct.

As for Choice B . . .

Technically, if you are discussing actual numbers, you should used "greater than" ---- 37 is greater than 24. My age is greater than the age of that teenager.

Use "more than" when you are discussing the quantity of object --- "Will the team score more points during this inning/quarter/period?", "Only one US President served for more than 8 years."

If we were counting gyrfalcons, we could say "there are more gyrfalcons now than when . . ."

Instead, we are talking about the pure number: "its numbers are now five times greater than when . . ."

Does that make sense? Please let me know if you have any questions about anything I have said.

Mike