The correct answer is A. Sunlight.
Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar. The energy from sunlight is absorbed by chlorophyll, a green pigment found in plant cells. This energy is used to split water molecules into hydrogen and oxygen. The hydrogen is then combined with carbon dioxide to create glucose, a type of sugar. The oxygen is released into the atmosphere.
Option B is incorrect because oxidation of glucose is a process that releases energy, not stores it. Option C is incorrect because the breakdown of proteins does not produce energy. Option D is incorrect because the breakdown of glucose does produce energy, but it is not the source of the energy stored in photosynthesis.