Answer
b) is correct.
Work Step by Step
The work done by force $\vec{F}$ on the object can be calculated by $$W=(F\cos\theta)s$$
The angle $\theta$ $\vec{F}$ makes with the object's motion decides the sign of $W$.
For $W\gt0$, $\theta$ should be in between $(0, 90^o)$. Here, we see that $\vec{F}$ can only make such angles with the $-x$ and $+y$ axis.
Therefore, the work done by $\vec{F}$ can only be positive when the displacement of the object is along the $-x$ or $+y$ axis.
