The answer is A. This is a statement of Newton's First Law of Motion.
Consider Newton's Second Law, which for constant mass says F = ma. If forces are balanced, F = 0. Therefore m*a = 0. Since m has a nonzero value, we find that a = 0.
The object is not accelerating when forces are balanced, i.e. it's velocity isn't changing. It is moving at a constant speed when this velocity is nonzero, and is stationary when the velocity is zero.
I hope this helps you :)
