This is the source of food for the new seedling until new leaves develop.

The cotyledon is indeed the correct answer because it serves as the first leaf or leaves that are produced by the seed of a plant. In the initial stages of growth, the cotyledon provides essential nutrients to the developing seedling until it is able to produce true leaves and photosynthesize on its own. This process is crucial for the establishment of the plant as it relies on the energy stored in the cotyledons for initial growth and development.

Other parts mentioned, such as the hypocotyl and epicotyl, play different roles. The hypocotyl is the part of the stem that is below the cotyledons and connects them to the root, while the epicotyl is the portion that will develop into the stem above the cotyledons. The embryo refers to the early developmental stage of the plant that is contained within the seed but does not perform the role of providing food to the seedling after germination; that role is primarily fulfilled by the cotyledon.