the same work is done lifting each bale, regardless of speed. A useful formula is W=mgh, which shows that as long as the bales have equal masses, the energy (work done) needed is the same for each bale. the question states the masses are equal, so we know work done per bale is constant.
As with anything, if you do it more quickly, more power is required, so when the rate of lifting bales goes up, so does the useful power. Thus the answer is D.