This is a method question and we are trying to understand how the argument goes about making its point.
The author basically says:
New hormone increases milk production. Some lawmakers say we should list all artificial substances used to make milk. Bad idea. If implemented, we will list thousands of items.
So D is tempting because it may feel like the discussion of synthetic fertilizers and fungicides is analogous, but really these are just instances used to prove what we would have to list if we did include every artificial substance. This list could go on forever because maybe the fungicides have many artificial components and so on, maybe the feed as well.
An analogy on the other hand would be clear. It would draw a similar situation. E.g. listing all of the artificial substances in a bag of chips.