40) For four elements the IP₂ curve is shown. In the graph of elements represented by A, B, C and D are.
$$\begin{gathered} 1) Ha, Mg, Al, Si \\ 2) Mg, Al, Si, P \\ 3) Al, Si, P, S \\ 4) Si, P, S, Cl \end{gathered}$$

Dear student

The correct option is b.

First ionisation                        2nd ionisation 
 Mg ⁺ = 2 8 1                             Mg²⁺ = 2 8
 Al⁺  = 2 8 2                              Al²⁺ = 2 8 1
 Si⁺  = 2 8 3                              Si²⁺ = 2 8 2
  P⁺  = 2 8 4                               P²⁺ = 2 8 3

After 2nd ionisation Mg attain a stable configuration so the ionisation potential of Mg is less than Al. 
The 2nd ionisation potential of Al is high because after the first ionisation, Al has a stable configuration and in 2nd IP electron ejects from a filled shell. Same for Si, it attains a stable configuration after 2nd IP.

The order of second ionisation potential  will be Mg²⁺ < Si²⁺ < Al²⁺ < P²⁺


Regards