Which substance within the seed is crucial for providing energy to the developing plant?

The cotyledon is indeed crucial for providing energy to the developing plant. Cotyledons are the first leaves that appear from a germinating seed and are vital in the early stages of a plant's growth. They serve as a source of stored food, often containing starches, proteins, and oils that are used by the embryo until it can produce its own energy through photosynthesis.

In contrast, the seed coat primarily serves as a protective outer layer, safeguarding the seed's contents from physical damage and pathogens. The endosperm, while significant as a nutrient source, is typically a temporary storage tissue in monocots, supporting seedling growth until cotyledons or true leaves can take over that role. Fruits, on the other hand, play a role in seed dispersal and protection but do not directly provide energy to the developing plant. Thus, the cotyledon's role as a primary energy source during the early growth phase makes it the correct answer.