Answer
(a) Electrons have been removed from the sphere and transferred to the rod.
(b) $2.5\times 10^{10}$ electrons were transferred.
Work Step by Step
(a) Since it is normally quite easy for electrons to move, we can assume that it is the electrons which are transferred when there is a transfer of charge. The rod's net charge changes by $-4.0~nC$. In this case, we can assume that electrons have been removed from the sphere and transferred to the rod.
(b) The change in the charge of the rod is $-4.0~nC$. Each electron has a charge of $-1.6\times 10^{-19}~C$. We can find the number of electrons that were transferred.
$\frac{-4.0\times 10^{-9}~C}{-1.6\times 10^{-19}~C} = 2.5\times 10^{10}~electrons$
$2.5\times 10^{10}$ electrons were transferred.
