You can "flag" an answer for moderator intervention. What are the flags, and which flag should I use?
The available/predefined reasons for flagging are:
See also How to not be a spammer
rude or abusive
People are usually polite and moderate, friendly, on this site.
But a post may be considered "rude or abusive" if it explicitly criticises and names another user, or a different school of Buddhism or even a teacher or public figure. Generally the questions on this site don't ask you to criticise others -- so, you can answer without doing that.
not an answer
This is for text which don't even try to answer to question, for example ...
I've had this problem too. Does anyone else have an answer?
... or ...
I don't understand this question!
very low quality
This is for answers like ...
lol, yes of course!
... or ...
Of course you should, why wouldn't you?
See also this meta-answer about what type of answer is conventional on this site.
Flagging is probably not applicable to answers which you consider wrong or untrue -- i.e. which are answers, but which you disagree with or consider "not useful".
Instead you may vote down this kind of answer (instead of flagging it).
When you flag an answer you are asking a moderator to delete it (or perhaps to edit it if it's rude or abusive). But moderators aren't supposed to delete an answer for being wrong or untrue, so just downvote this kind of answer instead of flagging. I think that means it's normal on SE for a few "wrong" answers to be left on the site -- but they should be downvoted.
irrelevant or off-topic answers
Sometimes an answer fails to address the topic of the question. If so I think that might be flagged as "not an answer".
This is also true if a question uses a tag like theravada to ask a question specific to that school, and an answer is based on doctrine of a different school.
This is the Buddhism.SE site so answers should be based on Buddhist doctrine and/or practice.
Answers based on other religions would be off-topic on this site, and should be deleted.
This is also true of a few answers which are based on "science" -- e.g. physics etc. -- not that Buddhism is anti-scientific, but people are asking questions about Buddhism not about science.