Please explain!

I struggled with this one too, but I think the keyword in the passage is "grew" (Line 37), which implies not naturally occurring, but rather, artificially created by the scientists.

in line 44, they talk about an accelerated evolution that "developed" additional "related" peptides....this strongly implies they created them (aka didn't exist before).

I would point to line 56, in which we are told they are developing new peptides.

I struggle to see why line 56 is the keyword at play for answer choice A. You are correct in saying that they did eventually develop new peptides after only seeing the promising results of the original peptides. I don't see much if anything else in the passage to support the fact that these peptides they originally tested were not naturally occurring.

I agree it's pretty soft, I would only have gotten to A through process of elimination. I think we can justify it by saying that if they were "designing new peptides" this implies that this is a process of active creation. While some of them might also occur in nature, to design new molecules does seem to imply that this is an aim to create truly new and unnatural peptides.