In a certain region of space, the electric field is zero. from this fact, what can you conclude about the electric potential in this region?

The answer is that it is constant. The relation between electric field and electric potential is given as, E=  -gradient (V).  The E, the partial rate of change of Electric potential, in the equation implies that the V, the partial differential of the potential of the three-dimensional space (assuming it is considered) is constant. 

It should be d if not then c