Read the question stem: Which of the following conclusions can be drawn ....
So options are conclusions. You are looking for a conclusion that you can draw from the argument. Remember that conclusions do not have any new information. They just infer from data given in the argument.
All B, C, D and E have new information in them which is not mentioned in the argument:
(B) Host insects lack any effective defenses against the form of predation practiced by parasitic wasps.
(C) Parasitic wasps learn from experience how many eggs to lay into the eggs of different host species.
(D) Failure to lay enough eggs would lead to the death of the developing wasp larvae more quickly than would laying too many eggs.
(E) Parasitic wasps use visual clues to calculate the size of a host egg.

Argument says:
- Parasitic wasps lay their eggs directly into the eggs of various host insects in exactly the right numbers for any suitable size of host egg. (e.g. if the host egg is 2 inches, they lay 1 egg in it, if it is 4 inches, they lay 2 eggs in it etc)

So we can infer that from the wasp's egg laying behavior (1 egg in 2 inch host egg etc ), the size of the smallest host egg can be determined. (It will be the one in which the wasp can lay only 1 egg - 2 inches in our example)