What happens when a question is answered, but the asker doesn't choose a solution? (or doesn't even return to the site since the question was asked). Is there a way to override this and mods can select the appropriate answer?
If it's possible, what is the time threshold for this?
Example: Does a person still need a ham radio license?
Was asked on the 27th Nov, and the last time the user logged on was the 29th.
This matter gets discussed on various meta sites from time to time, and as Amber pointed out a moderator cannot force-accept an answer. It wouldn't make sense either, as accepting an answer is about the original poster's opinion of which answer was most helpful whereas votes is about how the community feels about the answers given (as well as other content on the site). For example, the Meta Stack Overflow question/feature request Force Accepted Answers on Questions by Inactive Users is explicitly status-declined. The discussion question Why do incorrect answers keep getting "accepted"? approaches largely the same issue from a somewhat different angle.
There are also times, though I don't think I've seen any on Amateur Radio yet (we shouldn't be surprised if we get some of those, however), where none of the answers really meet a reasonable set of criteria for acceptance; for example, if an answer does not address all the concerns raied by the OP in the question, in my personal opinion it shouldn't be accepted unless there are some other, very compelling reasons why it should be.
Another possibility is that the OP is giving the rest of the community a chance to voice their opinion by posting alternative answers. The idea of Stack Exchange isn't to ask a question, get one answer, accept it and move on; the idea is to get multiple answers from different people, the community voting on the answers causing the best answer(s) to float to the top of the pile, and the asker at some point making a determination which answer helped him/her the most and accepting that one. I always suggest waiting at least 24 hours (longer on less active sites such as this one) before accepting an answer, simply to give others a "chance" as well. For example, that's the case with my own question What information should be on a QSL card?.
Remember that clicking on the "accept" checkmark is basically a way of telling the community "this answer properly addresses all my concerns and/or is the one that most helped me solve my problem". In general, questions which already have an accepted answer receive less attention from the community, which is actually precisely what we don't want on what is still a very young site. (There's nothing saying you cannot post another answer to a question that already has an existing, accepted answer, but it happens a lot less than with questions that do not yet have an accepted answer.)
If you post an answer, especially if the OP says (for example in a comment) that it was helpful and even more so if the OP is a regular visitor on the site (you can check the user's profile by clicking on their name), by all means leave a comment saying that if the answer was instrumental in solving their problem then they should indicate so by upvoting and/or accepting, as appropriate. But you cannot in any way demand that an asker accepts an answer, much less any particular one.
Bottom line, like Amber said, even if we as diamond moderators could accept an answer on behalf of someone else, we shouldn't and wouldn't. It's just not what answer acceptance is for and about; acceptance is about usefulness to the original question author, not someone else. And not every single question needs an accepted answer either, even though it's always nice to see that green checkmark next to an answer of one's own.
Mods cannot "force-select" answers (nor would we). The checkmark is there to indicate "solved the question-asker's problem" - something which only the question-asker can really judge.
Reputation is already granted based on upvotes; the extra rep from answer acceptance is just that - an extra.
