Identify the reaction/process in which the entropy increases.

Entropy is the measure of disorder or randomness. Since entropy is a state function, the entropy change of a system depends only on the initial and final state irrespective of the path taken. Gases have more degree of randomness than liquids and solids. liquids have more degree of randomness than solids. 

In the first reaction, two gas molecules becomes one gas molecule, hence, entropy decreases. In B and D options, gas is converting to solid. Hence, change in entropy is negative. In option C, liquid is changing to gas. i.e., entropy increases.