My first response would be do no harm (until positively confirmed), so select Ignore, the only other option.
Click the Advanced option and I believe there should be an option to submit the file to the avast virus lab for further analysis (or words to that effect).
Don't check the 'Do not tell me about this rootkit in the future' option, as I don't know if there is a way to reverse that and you want to know when avast no longer alerts on it (which you won't if you selected that option).