Answer: Combination of C and A.
Explanation: As the gases are heated, the velocity of the gases will increase, now, there is something called "escape velocity".
This "escape velocity" is the velocity needed for an object to escape the planet gravity. This phenomenon, where gases are heated and they scape into space, is called a hydrodynamic escape.
There is also Jean's escape, which actually describes how single particles reach the escape velocity.
Then, you can see that both things have an impact, if the gravity is low enough, the escape velocity will be less, and the particles will need less velocity to escape.
If the temperature is big enough, the velocity of the particles will be big enough to escape from the atmosphere.
In our particular case, option C has a bigger impact, because we first need to have an increase in temperature in order to see the effects of the option A.