Can't be B as it can and does relate to 2 engine aircraft also.
Can't be D as if you are in straight flight and the plane is trimmed etc then there is no reason why it shouldn't stay in straight flight as long as it doesn't slow to much and reach stall speed. As you say wouldn't want to try it!
Unlikely A as VMCA is not do with Climb gradient.
I would have said C anyway but by Elimination I would say C also.