[Vikesrock]

Alright, I've seen enough answers that I think are wrong to make begin to wonder if it is me that's wrong so here's what I got:

To get the farthest kick we want to impart the maximum kinetic energy on the ball. Assuming all the potential energy in the spring is converted to kinetic energy when firing, this means we want to maximize the potential energy stored in the spring.

F=kx
E = 1/2*kx*x

F in this case is the max motor force which is constant, therefore kx is constant. If kx is constant then E is proportional to x, therefore we want to maximize x.

Going back to the first equation we now know F (the max force the motor can apply to the spring based on gearing and lever arm) and X (the max spring extension before reaching the mechanical limits) so we can solve for k.

For x to increase, k will have to decrease so my answer is B, less spring rate.