What is the answer of jazakallah?

First of all, the correct statement is Jazaak Allahu Khairan. If you do not ad the Khairan, then what you are saying is either may Allah reward you, or may Allah punish you.

Khairan means "good", so you are asking that Allah rewards a person with good.

But the Word Jazaa' alone can be for reward or punishment, as it simply means "recompense".

The proper response to Jazaak Allahu Khairan (May Allah reward you with good)is: Wa Iyyaak ( and to you).