"Jocund" means cheerful, merry, and lighthearted. This question asks for the word most opposite to that meaning, not a word close to it.
Since jocund describes a cheerful mood, its true opposite must describe a sad one, which is "Melancholic."
Let's summarize:
So the correct answer is "Melancholic."
If the price of a commodity increases by 25%, by what percentage should the consumption be reduced to keep the expenditure the same?