It's B or C generally, but sometimes fixing on a single option might turn out to be risky. Instead of fixing upon one option, you could teach them to make an "intelligent" guess in multiple choice options. Since sometimes, some options we can easily discard comparing with the other options. You can check out a nice descriptive strategy I came across in the source link below.
If the test includes true or false questions as well or some questions with only 2 options, I'd say to suggest them option B since even if by mistake if they mark it C, it would be a pity. <Even random guess would have worked better!>
So if at all you still didn't want to incorporate intelligent guess making, I'd suggest B.
It is good that your are giving your students this extra angle which is generally missed out.