The correct answer is a) first trophic level.
Photosynthetic organisms, such as plants and certain types of algae, are classified as producers. They are capable of converting sunlight into chemical energy through the process of photosynthesis, forming the base of the food chain.
In an ecosystem, the first trophic level consists of these producers. They provide energy for all other trophic levels, which include primary consumers (herbivores in the second trophic level), secondary consumers (carnivores in the third trophic level), and so on. Therefore, all photosynthetic organisms fundamentally occupy the first trophic level, making them essential for the energy flow within any ecosystem.