Hi Never mind. It's used to tell someone not to worry about something because it's not important.However, not at all is a polite reply after someone has just thanked you.
- "Thanks for helping".
- "Not at all".
So never mind is right here.
Given the two choices, "never mind" would be the most appropriate response. "Not at all" would be appropriate as a response to a question such as, "Are you upset/angry/concerned that I have forgotten.........?"