Option 1: Combustion is not the process by which solar energy is converted into chemical energy. Combustion is the process of burning a fuel to release energy, typically by combining it with oxygen. Solar energy is not directly involved in this process.

Option 2: Transition is not the correct answer. The term transition does not specify any process by which solar energy is converted into chemical energy.

Option 3: Photosynthesis is the correct answer. In photosynthesis, plants use sunlight energy to convert carbon dioxide and water into glucose (a form of chemical energy) and oxygen. This process occurs in the chloroplasts of plant cells.

Option 4: Fusion is not the correct answer. Fusion is a nuclear process where two atomic nuclei combine to form a heavier nucleus, releasing a large amount of energy. Solar energy is produced in the core of the sun through fusion reactions, but this is not the process by which solar energy is converted into chemical energy in organisms.

Therefore, the correct answer is option 3: Photosynthesis.