Step 1: Understand the meaning of the given word.
"Glum" describes someone who looks or feels dejected, sullen, and low in spirits, the kind of mood you see on a person's face after bad news.
Step 2: Decide what the antonym should mean.
The opposite of a dejected, low mood is a bright, happy, and light-hearted mood.
Step 3: Eliminate the options that do not fit.
(a) Desire is a noun meaning a wish or craving, it is not even an emotion describing mood, so it does not fit the antonym slot.
(c) Sour describes a taste, or figuratively a bad-tempered, bitter attitude, which is actually close in feeling to glum, not opposite to it.
(d) Gloomy is a near synonym of glum, both mean dejected or dark in mood, so this cannot be the antonym.
(e) Respond is a verb meaning to answer or react, completely unrelated to mood.
Step 4: Confirm the best match.
(b) Cheerful means noticeably happy and optimistic, which is the direct opposite of glum's gloominess.
Step 5: State the answer.
The correct answer is option (b), Cheerful.