Scientist are studying photosynthesisIn in a forest ecosystem that has plants animals and decomposers

Ecosystem is food web of primary and secondary producers and consumers.

Explanation:

In an ecosystem, there exist primary producers, primary consumers, secondary consumers, and tertiary consumers such as detritivorous such as viruses and bacteria that act on the living bodies and recycle the nutrients back to plants through the soil. The primary producers such as all green plants and others make their own food being autotrophs they use sunlight and carry out photosynthesis process. On the depend on the herbivorous they eat grass etc. The secondary consumers are carnivores as heterotrophs that feed on herbivores.

The answer to this is the 3rd choice!