Correct Answer - B
All these elements belong to the `3rd` period of the periodic tavble. In general, the ionization enthalpy increases acorss the period on account of the increase of effective nuclear charge. However, `A1(3s^(2) 3p^(1))` has lower `IP` than `Mg(3s^(2))` because it is easier to ionize a `p` electron than an `s` electron from the same shell. `P(3s^(2)3p^(3))` has higher `IP` than `S (3s^(2)3p^(4))` as it is relatively difficult to ionize the extra stable half-filled subshells.
