Answer
In situation (a), the object could be in equilibrium.
Work Step by Step
(a) 3 forces pointing along the same line but in different directions could cancel each other out and lead to $\sum F=0$. That means a no-acceleration case is possible and the object may be in equilibrium.
(b) 2 perpendicular forces can never cancel each other out. Take $F_1$ and $F_2$ perpendicular with each other. We have $$\sum F=F_1+F_2=F_1+F_1\cos90=F_1+0=F_1\ne0$$
Thus, there is always acceleration available and the object cannot be in equilibrium.
(c) A single force acting on the object has no other forces canceling it out. Therefore, $\sum F=F\ne0$ and the object cannot be in equilibrium.
