Kin selection explains helping relatives, and reciprocal altruism explains helping those who may repay it. Why does anonymous helping of a stranger you will never meet again resist both explanations?

D. Because neither mechanism has anything to work on here.

Both evolutionary accounts have prerequisites anonymous one-off helping removes: no relatedness for kin selection, no prospect of return for reciprocity. What remains is different in kind, internalised norms, empathic concern, and the mismatch argument that genuine anonymity was rare for most of human history, so a disposition that paid on average still fires.