Answer
The work done by the electric force due to Sphere B is negative.
Work Step by Step
We can write a general equation for the electric potential around a point charge:
$V = \frac{1}{4\pi~\epsilon_0}~\frac{q}{r}$
Sphere B is positively charged so $V \gt 0$ at all points.
As $r$ decreases, the magnitude of $V$ increases.
Therefore, sphere A would be moved to a higher electric potential.
That is $~~V_f \gt V_i$
We can find an expression for the work done by the electric force due to Sphere B:
$Work = -\Delta U = -(q_A~\Delta V) = -q_A~(V_f-V_i)$
$q_A \gt 0~~$ and $~~(V_f - V_i) \gt 0$
Therefore, the work done by the electric force due to Sphere B is negative.
