c
According to Dalton's law of partial pressure

\(p _{ i }= x _{ i } \times P _{ T }\)

\(P _{ i }=\) partial pressure of the \(i ^{\text {th }}\) component

\(x _{ i }=\) mole fraction of the \(i ^{\text {th }}\) component

\(p _{ T }=\) total pressure of mixture

\(\Rightarrow 2 atm =\left(\frac{ n _{ H _{2}}}{ n _{ H _{2}}+ n _{ H _{ e }}+ n _{ O _{2}}}\right) \times p _{ T }\)

\(\Rightarrow p _{ T }=2 atm \times \frac{3}{1}=6 atm\)