Answer
b) cannot be true.
Work Step by Step
Since the object is moving at a constant velocity, it has no acceleration, which means $\sum F=0$
$\sum F=0$ when either no forces are acting on the object or all the forces acting on the object have balanced one another.
In b), when a single force $F$ acts on the object, it is not balanced by any other forces, so $\sum F=F\ne0$, so b) cannot be true.
