Answer
$b=c$ and both are greater than $a$
Work Step by Step
The shell theorem states that when a charged particle that has a uniform charge around its surface inside a shell does not have a net force.
And when a charged particle that has a uniform charge around its surface outside a shell experiences a force as if the shells charge were concentrated as a particle in its centre.
a) Has no net force
F= 0
b)$F= \frac{4Q\times 2q}{d^2} = \frac{8Qq}{d^2}$
c) = $\frac{8Q\times q}{d^2} = \frac{8Qq}{d^2}$
b=c and both are greater than a.
