Initial force between 1 and 2 is `F=kq^(2)//r^(2)=88mN`. Charge on 1 after 3 is touched with 1 is `q//2`. Same charge will be on 3 also. Charge on 2 after 3 is touched with 2 is
`(q+(q//2))/(2)=(3q)/4`
Now, force between 1 and 2 in situation (d) is
`F=(k(q//2)(3q//4))/(r^2)=(3 kq^(2))/(8r^2)=3/8xx88=33mN`.