(B) is trading on a confusion between “genetic difference” at different levels. Here’s what I mean.
When you say of two individual animals that they are “genetically different” that actually doesn’t tell you much. Of course they’re “genetically different.” Only clones and maternal twins are “genetically identical.” Brothers, cousins, your bff, etc. are all “genetically different.” We're all genetically unique.
That’s at the individual level.
When you move up to the group level, “genetic difference” tells you quite a bit more. If we were observing a large clan of say 150 members and we randomly split them into two groups of 75 members each. You would say of the two groups that they were NOT “genetically different.” Of course not, they’re of the same clan. Certainly no one would interpret that statement to mean we're talking about 150 clones. Instead, the understanding is that while they are 150 genetically unique individuals, they share enough genes such that when describing them as two groups, we say that they are NOT "genetically different."
On the flip side, when you say of groups that they are “genetically different” (or substantially so) then you’re saying that they probably did not interbreed.
The stimulus uses that term on the group level whereas (B) uses that term on the individual level.