Answer
The charge on the inner surface of the conductor is $~~+1.2\times 10^{-9}~C$
Work Step by Step
In part (b), we found that $Q = -1.2\times 10^{-9}~C$
Since the electric field inside a conductor is zero, if we draw any Gaussian sphere within the conductor with a common center, the enclosed charge must be zero.
Therefore, the charge on the inner surface of the conductor must be $~~+1.2\times 10^{-9}~C$
