Answer
Please see the work below.
Work Step by Step
(a) We know that the depth of the sink with water is reduced by a factor of n than it is empty.
As $d_{water}=\frac{d}{n_{water}}$
$d_{water}\lt d $ because $n_{water}\gt 1$
Thus, the sink with water appears to be shallower.
(b) We know that the best explanation is option (II) -- that is, water bends the light, making an object under the water appear to be closer to the surface. Thus, the water-filled sink appears shallower.
