Electron configuration of Al atom can be given as follows.
`Al = 1S^2,2S^2,2P^6,3S^2,3P^1`
Most common form of Al ion is `Al^(3+)` . Because it has the electron configuration of Ne which is a inert element. The electron configuration of `Al^(3+)` is given below.
`Al^(3+) = 1S^2,2S^2,2P^6`
To form `Al^(3+)` one 2P electron and two 2S electrons will be removed. When we come to `Al^(4+)` we have to remove another electron from the outer sub shell. So the outer sub shell is 2P.
So the answer is 2P sub shell.Answer C.