Here we are talking about the number of antibodies, not the antibodies themselves.
Note the last part of the sentence ...
"...targeted at an invading microbe or foreign substance."
The gist of the sentence is that the genes are able to generate a number of antibodies of different types.
With the use of "all", the meaning changes to imply that all the types of antibodies are targeted towards a single microbe or foreign substance.
It leads to ambiguity, whether the genes generate antibodies of different types for a single type of microbe or a single type of antibody for a particular microbe.
I got this in the practise test today and narrowed it to D/E & picked D under severe time pressure. However now I see why E is the correct answer.
Haas can you explain why each is correct. Isnt antibodies plural and so we should pick all?