Answer:
Have not
Explanation:
Have not combines to become haven't. For example, I have not ate food today you can also say I haven't ate food today.
Cannot is formal, while can't in informal.
"Cannot" and "can not" are interchangeable, but when you are asking question for example:
"why can't I...", you can also say "why can I not..." (which is more formal), but you can't say "why cannot I..."
