Breakdown of Czym pan się interesuje? — Historią.
pan
sir / Mr.
interesować się
to be interested in (+ Instrumental)
czym
with what, in what, by what (Instrumental of co)
historia
history
Questions & Answers about Czym pan się interesuje? — Historią.
Why do we use czym instead of co for the word "what"?
The verb interesować się (to be interested in) always requires the thing you are interested in to take the instrumental case. Czym is the instrumental form of the basic word co (what).
Why is the verb interesuje in the "he/she/it" form if the English translation says "you"?
When speaking formally to a man, Polish uses the word pan (sir) combined with the third-person singular verb form. You are literally asking "What is the gentleman interested in?".
What is the purpose of the word się in this question?
It is a reflexive pronoun, similar to "oneself". The Polish verb for "to be interested" is interesować się. If you drop się, the verb interesować just means "to interest" someone else.
Why does the answer say historią instead of just historia?
Even though it is a one-word answer, it acts as a shortcut for the full thought: "I am interested in history". Because the verb requires the instrumental case, the feminine noun historia must change its -a ending to -ą.