Final answer:
The correct term for the underside of a floor that shows supporting beams and is finished to form a ceiling is 'ceiling'. It conceals the structural components such as joists and beams and is integral to interior design.
Step-by-step explanation:
The term used to describe the underside of a floor that shows the supporting beams and which is finished to form a ceiling is ceiling. This architectural feature is found commonly in homes and buildings, and it serves both a functional and aesthetic purpose. The ceiling covers the structural elements like joists and beams, and is usually part of the building's interior design.