Consider the elements Mg, Al, S, P and Si, the correct increasing order of their first ionisation enthalpy is:

We need to arrange Mg, Al, S, P, and Si in increasing order of their first ionisation enthalpy.

These elements belong to the third period. The general trend is that ionisation enthalpy increases across a period from left to right. However, there are two well-known exceptions in the third period.

First, Al (group 13) has a lower ionisation enthalpy than Mg (group 2). This is because Mg has a fully filled 3s orbital ($$3s^2$$), which is extra stable. Al loses an electron from the 3p orbital, which is easier to remove.

Second, S (group 16) has a lower ionisation enthalpy than P (group 15). This is because P has a half-filled 3p orbital ($$3p^3$$), which is extra stable. S has one paired electron in a 3p orbital, and the electron-electron repulsion makes it easier to remove.

So the correct increasing order is: $$Al < Mg < Si < S < P$$.

Hence, the correct answer is Option 1.