The last sentence shows that different allergic people respond differently to different cats. That would not be true if all cats were identical with respect to the proteins found in their skin secretions, since those are what cause allergic reactions. You can essentially reason out C by using the fact that the contrapositive of a conditional statement is logically equivalent to the original statement:
*People who are “allergic to cats” are really allergic to certain proteins (first sentence)
*If the cats all produced the same proteins, allergic people would be allergic to the same cats (follows from first sentence)
* Allergic people are NOT all allergic to the same cats (last sentence); this starts to the contrapositive of bullet point 2, directly above.
*Therefore, cats don’t all produce the same proteins (continuation of the contrapositive; also answer choice (C)).
It’s common for a given cat to cause a reaction in some, but not all, people who are allergic to cats (last sentence); however, that doesn’t rule out the possibility that the might be a cat or two that cause reactions in all allergy sufferers. Since we can’t rule that possibility out, (B) is wrong; it COULD be true, but (C) HAS TO be true.