Answer: The correct option is (a).
Step-by-step explanation:
If two gases are present in a container, the total pressure in the container is equal to the sum of the all the individual pressures exerted by the individual gases in a container.
Individual pressure exerted by the individual gas in a container is called partial pressure of that individual gas.
So, total pressure of gas in container is:

Where
= Total pressure exerted by the gases in the container.
= Partial pressure of gas 'a'
= partial pressure of gas 'b'