？
How did you reply?
Literal
What [quotative-って] answered [explanatory-question-の]?
何て is the casual contraction of 何と — the quotative と attached to a question word, used here to ask 'with what (words) did you respond?' Specifically, the て-form of the quotative って is even more conversational than と. Compare 何と言った? ('what did you say?'), 何て答えた? ('what did you answer?').