The correct answers are: In this realm, the wealthy live in floating castles while the poor dwell in the caves below. The elderly are highly respected for their knowledge of ancient spells.

We can use "the" + an adjective to talk about a specific group of people in society (e.g., the wealthy, the poor, the elderly, the brave).

When used this way, the expression is always plural, but we never add an "-s" to the adjective. Therefore, "the braves" and "the poors" are grammatically incorrect.