(c)
According to Raoult’s law
$\frac{p - p_{s}}{p} = \frac{n}{n + N} = \frac{0.05}{2.5 + 0.05}$
$= \frac{0.05}{2.55} = \frac{1}{51}$
weight of solute $= \frac{w}{W} \times M \times \frac{p}{p - p_{s}}$
= $\frac{10 \times 18}{90} \times 51$
=102 g