Answer
c) is correct.
Work Step by Step
The work-energy theorem states that $$W=\frac{1}{2}m(v_f^2-v_0^2)$$ $W$ is the work done by a net external force on the object
We have $$v_f^2-v_0^2=7^2-15^2\lt0$$
Therefore, $W\lt0$, so the force has done negative work on the ball.
