You should eliminate (A) at the word motives, which points to a different (and often flawed) pattern of reasoning – the argumentum ad hominem. It’s a common pattern, so you should aspire to recognize it immediately when you see it in the stimulus or answer choices.

The new considerations the vet cites might suggest that on balance the breeders’ conclusion isn’t well supported by their evidence. But that falls short of inconsistency.

Inconsistent is a very strong word. You need two claims that cannot possibly both be true. It’s almost always wrong to accuse an LSAT argument of being inconsistent.

Inconsistent isn’t just “Parties are Bill’s favorite activity, but he hasn’t been to a party in ages.” Inconsistent is “Parties are Bill’s favorite activity, but parties are not Bill’s favorite activity.”

The vet’s evidence weighs against the horse breeders’ evidence, but falls short of contradiction. Contradictory evidence would be have to show the disease doesn't kill horses.

Contradicts is a very strong word. You need two claims that cannot possibly both be true.

A contradiction isn’t just “Parties are Bill’s favorite activity, but he hasn’t been to a party in ages.” A contradiction is “Parties are Bill’s favorite activity, but parties are not Bill’s favorite activity.”

The vet doesn’t contest the genetic defect’s downsides. They don’t say “actually the disease doesn’t kill horses!”

The vet acknowledges those downsides, but brings up additional factors that potentially outweigh those downsides.

The vet does indeed introduce considerations (diet and beauty) that lead to a different conclusion (they are wrong / let’s breed some diseased horses) from that of the horse breeders.