Explanation

(d)
In Ist case,
When two liquids X and Y are mixed in the molar ratio 1:1.
Moles of X = 1
Moles of Y =1
Mole fraction of X ($ϰ_{x}$)=$\frac{1}{2}$
Mole fraction of Y ($ϰ_{Y}$)= $\frac{1}{2}$
We know that
p=$p_{X}^{\circ}ϰ_{x} + p_{Y}^{\circ}ϰ_{Y}\$ (p=total pressure of mixture)
$400 = \frac{1}{2}p_{X}^{\circ} + \frac{1}{2}\ p_{Y}^{\circ}$
$\ \ \ 400 \times 2 = \ p_{X}^{\circ} + p_{Y}^{\circ}$ …(i)
For case II^{nd} ,
When liquids are mixed in the molar ratio of 1:2,
Mole fraction of X =1
Mole fraction of Y = 2
Mole fraction of X ($ϰ_{x}$) =$\frac{1}{3}$
Mole fraction of Y ($ϰ_{y}$) =$\ \frac{2}{3}$
$P = p_{X}^{\circ}ϰ_{X} + p_{Y}^{\circ}ϰ_{Y}\$
$350 = \frac{1}{3}p_{X}^{\circ}\frac{2}{3}\ p_{Y}^{\circ}\$
$350 \times 3 = p_{X}^{\circ} + 2p_{Y}^{\circ}$ …(ii)
From Eqs (i) and (ii) , we get
$p_{X}^{\circ} = 550mm$
$p_{Y}^{{^\circ}} = 250\ mm$