Step 1: Understand the meaning of JOCUND.
Jocund means cheerful, merry, and lighthearted, full of high spirits and good humor, as in a jocund mood or a jocund gathering.
Step 2: Find the opposite meaning.
Since jocund describes a bright, happy state of mind, its opposite must describe a low, sorrowful state of mind. Melancholic means sad, gloomy, and prone to a pensive, low mood, which is the direct emotional opposite of being jocund.
Step 3: Eliminate the other options.
Brash means bold, impudent, or hasty in a rude way, a trait about manner, not about mood polarity. Jocular is actually close in meaning to jocund itself, since it means fond of joking or humorous, so it is a near-synonym trap rather than an antonym. Square, used informally, means old-fashioned or overly conventional, which does not address the cheerful-versus-sad axis at all. Wild means uncontrolled or unruly, again not a description of mood in the happy-sad sense.
Step 4: Conclusion.
Melancholic, option (d), is the correct antonym of jocund.