Is there a polite way to ask a question giving 2 alternatives (a) and (b) as possible answers, and avoid those funny guys who insist in answer with a new non-given alternative (c)?
(a) Yes, there is. (Please elaborate)
(b) No, there is not. (Please elaborate)
Yes, there is a way: Address C preemptively in the question yourself, and explain why it won't work for you.
If C is such an obvious choice, as a community, Stack Overflow wants and needs that C answer there in the question, with votes, because Stack Overflow is as much as about leaving good artifacts for future searchers as it is about solving your immediate problem. If there is an obvious C answer, the question needs to address it somewhere, whether it be in the question text or in an answer.
Additionally, as the question asker you are admitting that you need a little help with the topic. It's perfectly reasonable for experts in the topic to want to make sure that you're not overlooking some obvious C answer that would greatly simplify your question. If you haven't already called out why C won't work for you, experts who ignore it wouldn't be doing a very good job answering the question.
StackOverflow is not about a poll of two options - it is about giving a good answer to a question.
By limiting people to what you think the right answers may be, you could be losing a much better option.
In general, if there are additional constraints, you should specify them in the question (and if they seem unreasonable, elaborate on them).
So, if you can explain why only a and b are options, then you have politely explained why these are the only options (though, you might be wrong...).
(a) Yes, there is.
Ask a clear question that was well-researched, wherein your A and B choices are actually good ones, to begin with.
By no means should you ever think that merely stating "Please only answer A or B" will work. It will often have the exact opposite effect.
My experience has been if you're avoiding option C, it's because you're focusing too much on option A or option B, and not on objective O.
I'm not saying go with the crowd just for the sake of it, but obviously you're having trouble and are reaching out to people of more experience, so why not listen to that experience? Chances are someone has been down this road before, and have decided that A and B seem like the better alternatives right now, but when you run into problem P and roadblock R, option C sounds pretty nice.
Of course you can always counter this by saying Mandate M and Manager PHB have decreed option C to be null and void (even in languages where those keywords don't exist), and there's no way for the answerer to know you're bluffing. This is deceitful and most certainly not in the spirit of StackOverflow.
Bottom line is If you're avoiding options for no reason, you're only hurting yourself as a problem solver, and missing an opportunity to learn.
Be careful that you are not falling victim to the XY Problem.
If you need help choosing between (usually) two options, but you are unable (or unwilling) to consider other solutions, it might be because you are approaching the problem from the wrong angle.
One of my own questions actually qualifies as a good example here:
Trying to add a duplicate to a set - what kind of exception to throw?
I initially titled it "Trying to add a duplicate to a set - UnexpectedValueException or OverflowException?", but I reconsidered before posting because I wanted to be open to the possibility that there was a third exception type that would be a better fit...
... and in the end, the solution was not to throw an exception at all.
If you know the answer is going to be either (a) or (b) but not (c), then perhaps you should not be asking the question in the first place. Instead your question should goes something like:
What is the comparative advantage of solution (a) versus (b)?
Stackoverflow is not about opinions but solutions. You should choose the option yourself based on the pros and cons provided.
