The word GLUM means sad, low in spirits, or looking dejected and morose. The question asks for the closest antonym, so we need the option that means the opposite of feeling low or sad.
The correct option is Cheerful, since it directly opposes the sad, low mood that glum describes.
Let's summarize:
So the closest antonym of GLUM is Cheerful.
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?