(c)
Mole fraction of A =$\frac{moles\ of\ A\ }{total\ moles\ }$
Given,
moles of Ar = 1, moles of ${CO}_{2}$ = 2,
moles of $O_{2}$ = 3, moles of $N_{2}$ = 4,
moles of $O_{2}$ removed = 1
Mole fraction of $O_{2}$ at initial stage
=$\frac{3}{1 + 2 + 3 + 4} \times$100=$\frac{3}{10} \times 100 = 30$
Mole fraction of $O_{2}$ at final stage
= $\left( \frac{3}{10} - \frac{2}{1 + 2 + 2 + 4} \right) \times 100\ \$
=$\left( \frac{3}{10} - \frac{2}{9} \right) \times 100 = \frac{70}{9}\ \cong \ 8$
$\therefore$ % change $= \ \frac{8}{30}\ \times \ 100\ = 26\%$