Hint :Entropy can be confused with energy sometimes but they both are two different quantities. Entropy is a state function. It is defined as the measurement of the degree of randomness (lack of pattern or predictability in events) of energy in the system. More the randomness of particles, more is entropy and vice-versa.
Complete Step By Step Answer: Entropy is the degree of randomness. It is denoted as ‘S’. Entropy increases in the manner S(solid) < S(liquid) < S(gas). Solids have the least entropy. Liquids have entropy a little higher than solids but lower than gases. Gas molecules have maximum entropy, that is their degree of randomness is maximum. This difference between the entropy is because of the arrangement of atoms and the randomness of atoms. Entropy is the maximum observed in gases because gas molecules have high kinetic energy and they keep on moving in random directions. Now when two gases are mixed they both possess molecules which have kinetic energy in them and so the molecules of both the gases will collide with each other, the collision per unit area will increase and hence the randomness of each molecule will increase tremendously. And thus when two gases are mixed their entropy is going to increase. Therefore the correct option is C. Increases.
Note : Mixing of gases is irreversible. When a solid reactant is changed to a liquid product the entropy increases. Even when any substance is broken down the randomness of particles will increase resulting in an increase in entropy. Solids molecules are closed and can hardly move, liquid molecules can move a little bit but gases can move freely in any direction. This is also the reason why gases have maximum entropy.