According to Dalton's law of partial pressure,
PA=χA×Ptotal
where, PA = Partial pressure of gas A
χA= Mole fraction of gas A
PTotal= Total pressure
The lighter gas will have a lower value of molecular weight.
Therefore, partial pressure of lighter gas =χlighter gas×PTotal
=x4x4+x40×1.1
=1×40×1.14×11
=1 atm
Hence, the correct answer is option C.