A solid conducting sphere of radius ra is placed concentrically inside a conducting spherical shell of inner radius rb1 and outer radius rb2. The inner sphere carries a charge Q while the outer sphere does not carry any net charge. The potential for rb1 < r < rb2 is:

The right answer is "".

Explanation:

As the outer spherical shell is conducting, so there is no electric field in side from

⇒ .

So the electric potential at all points inside the conducting shell that from

⇒  

and will be similar as well as equivalent to the potential on the outer surface of the shell that will be:

⇒

Thus the above is the right solution.