Your answer would be C, "By conserving energy"! A gas can do work through something called the First Law of Thermodynamics. Heat that energy used by the gas during this "work" cannot be created or destroyed however it can be transferred and converted. This energy can only be stored during these processes, not removed. An example of this would be gas in a cylindrical object such as a piston. When the gas is heated, it expands and moves the piston upwards. When it expands, it stores that energy it gains through the heat given.
I hope this helps! (: