Volume of cylinder is constant
$P_1V = n_1RT$
$P_2V = n_2RT$
Or $\large\frac{P_1}{P_2} =\large\frac{ n_1}{n_2} = \large\frac{\large\frac{w_1}{m}}{\large\frac{w_2}{m}}$
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;=\large\frac{w_1}{w_2}$
$\large\frac{10}{8} = \large\frac{15}{w_2}$
$w_2 = \large\frac{15\times8}{10}$
$w_2 = \large\frac{120}{10}$
$w_2 = 12kg$
$\therefore$ Gas leaked = 15-12
$\;\;\;\;\;\;\;\;\;\;\;\;\;\;\;=3kg$
Hence answer is (c)