[The question is a common one in his time so the idea that it would be offensive never even crossed Jean-Luc's mind. Although usually the different species had specific markings, sometimes through dilution of traits or inter-breeding those sorts of things eventually disappeared altogether or weren't as obvious.]
A mutant. Implying that your abilities are a mutation on human DNA as we know it. [It's said as a statement but his curiously arched brow implies the question in said statement.]