Let the tension in the cord be $T$. The block is moving downwards with a constant acceleration $a = \frac{g}{4}$. Applying Newton's second law of motion to the block in the downward direction: $Mg - T = Ma$ $T = M(g - a) = M\left(g - \frac{g}{4}\right) = \frac{3Mg}{4}$

The force exerted by the cord (tension $T$) is directed vertically upwards, while the displacement $d$ is directed vertically downwards. The angle between the force and displacement vectors is $\theta = 180^\circ$. The work done by the cord on the block is given by: $W = Td \cos 180^\circ = -Td$ $W = -\left(\frac{3Mg}{4}\right)d = -\frac{3Mgd}{4}$