A — Cotyledon
The cotyledon stores food and provides nourishment to the embryo during germination of the seed.
Source: How Do Organisms Reproduce, Chapter 7
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The cotyledon (seed leaf) is the food-storing part of the seed. During germination, the embryo (radicle → root; plumule → shoot) draws nutrition from the cotyledon until the seedling can photosynthesize. Radicle and plumule are parts of the embryo that grow into root and shoot respectively — they are not food sources. "Embryo" is too broad and is the developing plant, not the food reserve.