I wanted to flag a question that was off topic because it was asking legal questions however there is no option for this. The closest, 'belongs on a stack exchange site' doesn't apply because as far as I am aware they all avoid these type of questions. Where is the option for plain off topic?
You're right, there is currently no generic 'off topic' flag to close reason. There is one available to use when you hit the 3000 reputation requirement for casting close votes though.
The reasoning for the generic Off Topic reason not being available for flag choices is outlined by Shog9 (Stack Exchange Community Manager) on the main Stack Exchange Meta site:
We'll revisit the need for a low-rep user "other" option based on usage, but initially we're trying to restrict the use of flag-to-close to problems that are generally agreed-on by the community.
If you see a post that should be closed for an unusual reason, you have the option of flagging for moderator attention, selecting "other" and describing that reason in detail.
As you can tell by the vote count on that post it doesn't seem to be the most popular suggestion for flagging OT usage though.
There are some further discussions on this in other posts on that Meta site, such as:
For now feel free to flag to close with whatever is the closes reason based on the topic of the question, and if it's totally Off Topic then flag it with a custom 'Other' flag for moderator attention and we'll look into that post to see if it needs to be moved / closed / deleted from there. Then just work on your rep so you can get to 3000 and start casting actual close votes of your own.