Final answer:
The source of energy for photosynthesis is sunlight, which excites chlorophyll molecules to start the process of converting light energy to chemical energy, eventually producing sugars and oxygen.
Step-by-step explanation:
The source of energy that drives photosynthesis is a) Sunlight. During photosynthesis, chlorophyll molecules in the photosystems absorb the energy from sunlight. This absorbed light energy is used to excite electrons which are provided initially by the splitting of water (H2O) molecules. These excited electrons then pass through the photosynthetic electron transport chain, eventually leading to the production of chemical energy in the form of ATP and NADPH. These molecules are then used to convert carbon dioxide (CO2) into sugars/carbohydrates.
Hence, sunlight not only fuels the process that leads to the production of sugars but also is the initial source of the electrons that run the electron transport chain in the chloroplast. Additionally, photosynthesis generates oxygen as a by-product which is vital for the survival of aerobic organisms, including humans.