In residential construction, building a concrete foundation directly on the ground is called:

A slab-on-grade foundation is a flat concrete slab poured directly on the ground, often used in warmer climates where ground freezing is not an issue. It supports the building structure directly and does not include a crawl space or basement. Pier and beam and post and beam refer to elevated structures, while platform construction refers to framing, not the foundation. Therefore, the correct answer is D.

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