(N/A) No. Current can be induced in a closed loop only when the magnetic flux linked with the loop changes with time. Since the loop and the magnets are stationary,the magnetic flux remains constant,and hence no current is generated,regardless of the strength of the magnets.
$(b)$ No current is induced in either case. Electromagnetic induction is caused by a change in magnetic flux,not electric flux. Since the electric field is constant and the loop moves normal to it,the electric flux through the loop may change,but this does not induce an electric current in the loop.
$(c)$ The induced emf is given by $\varepsilon = Blv$,where $l$ is the length of the side of the loop cutting the magnetic field lines. For a rectangular loop,the length $l$ perpendicular to the velocity $v$ remains constant as it exits the field,so the induced emf is constant. For a circular loop,the length of the chord cutting the field lines changes continuously as it exits,so the induced emf varies.
$(d)$ According to Lenz's Law,the induced current will oppose the cause of its production. As the magnets move towards the loop,the magnetic flux through the loop increases. The induced current will create a magnetic field to oppose this increase. Using the right-hand rule,the induced current will flow in a direction such that plate $A$ becomes positive with respect to plate $B$.