The correct answer is - A. Photosynthesis.
Through the process of photosynthesis, the plants manage to use the sunlight, and combining it with the nutrients that they have got through their root system, they manage to produce glucose. Then they use the glucose by converting it to energy, which is crucial for their survival and proper growth, especially for their fruits, vegetables, seeds, spores.
This process is only able to occur with enough sunlight at disposal for the plant, and even if there's very small amount of nutrients gathered from the soil or water, the plants are still able to perform this process.