Lindsey: There are, of course, many poets with cheerful dispositions; however, those I have met have much more often been disposed to melancholy.
Conclusion (Thus / conclude)
If the poets I've met are representative of poets generally, one can reasonably conclude that many poets are made melancholy by writing poetry.
Evidence
The poets I have met are more often disposed to melancholy, and an activity as profound and engrossing as writing poetry can be depressing.
Evaluate
The author supports his conclusion that "writing poetry makes many poets melancholy" with two pieces of info: 1. the sample of poets he knows mostly include melancholy poets 2. there's a reason why there would be a plausible causal path from writing poetry to being melancholy (poetry --> profound and engrossing activity --> can be depressing --> melancholy).
Goal
The question stem is asking us about the final claim, with is a Supporting Idea. It supports the middle sentence, which is the conclusion.