A solid spherical conductor has charge +Q and radius R. it is . . .

Question: A solid spherical conductor has charge +Q and radius R. it is surrounded by a solid spherical shell with charge -Q, inner radius 2R, and outer radius 3R. Which of the following statement is true? 

(a) The electric potential has a maximum magnitude at C and the electric field has a maximum magnitude at A.

(b) The electric potential has a maximum magnitude at D and the electric field has a maximum magnitude at B. 

(c) The electric potential at A is zero and the electric field has a maximum magnitude at D.

(d) Both the electric potential and electric field achieve a maximum magnitude at B.

Solution : 

We know, 
If a conducting body is given a charge then the charge will appear on the outer surface of the conductor. So the outer surface of the solid spherical conductor will have +Q charge. 

The inner surface of the spherical shell will get induced charge of -Q due to the +Q charge on the solid spherical sphere. 
Consequently, the outer surface of the spherical shell will also get an equal and opposite charge of +Q but as it is already given in the question that the spherical shell is also given a -Q charge. So the net charge on the outer surface will be zero (+Q-Q=0)

Also
AB=R
AC=2R
AD=3R

Now, the corresponding Electric Field and Electric Potential at points A, B, C and D respectively will be 

Clearly, after all the consideration, (d) would be the correct option.