Historically, "he" was generally used in the absence of a gender-neutral pronoun. But in a time of increasing political correctness, he can come across as presumptuous. The pronoun "it" is genderless, but using it to refer to people would likely be considered offensive. "One" is also used, but in conversational English, it sounds a bit silted and formal.
If you cannot reword the response to avoid playing the pronoun game, use the word "they".
From English Language & Usage: Gender neutral pronoun:
Singular they enjoys a long history of usage in English and can be used here: "Each student should save their questions until the end.". However "singular they" also enjoys a long history of criticism. If you are anxious about being criticized (for what is in fact a perfectly grammatical construction) you are advised to reword to avoid having to use a gender neutral singular third-person pronoun.