Let the mass of methane and oxygen is $w$
mole fraction of oxygen $=\frac{\frac{w}{32}}{\frac{w}{32}+\frac{w}{16}}$$=\frac{\frac{1}{32}}{\frac{1}{32}+\frac{1}{16}}=\frac{\frac{1}{32}}{\frac{3}{32}}=\frac{1}{3}$
Let the total pressure be $P$
The pressure exerted by oxygen (partial pressure) $=X_{O_2}\times P_{total}$$\Rightarrow P\times\frac{1}{3}$
Hence, (3) is correct.