Answer
We can rank the spheres according to their volume charge density:
$(a) \gt (b) \gt (c) \gt (d)$
Work Step by Step
We can write a general expression for the volume charge density:
$\rho = \frac{charge}{volume}$
Since the charge $Q$ is the same for all four spheres, a sphere with a smaller volume will have a greater volume charge density.
We can rank the spheres according to their volume charge density:
$(a) \gt (b) \gt (c) \gt (d)$
