A cord is used to lower vertically a block of mass $M$, a distance $\mathrm{d}$ at a constant downward acceleration of $\mathrm{g} / 4 .$ The work done by the cord on the block is:
(a) $M g d / 4$
(b) $3 \mathrm{Mg} d / 4$
(c) $-3 M \mathrm{~g} d / 4$
(d) $M \mathrm{~g} d$