Food chains are a representation of the flow of energy and nutrients through an ecosystem. They depict the feeding relationships between organisms, with each organism occupying a specific trophic level. The correct sequence of a food chain is as follows:

1. Producers (Autotrophs):
Producers, also known as autotrophs, are the foundation of any food chain. These organisms have the ability to produce their own food through photosynthesis or chemosynthesis. They convert sunlight, water, and carbon dioxide into glucose and oxygen, providing energy for all other organisms in the food chain.

2. Primary Consumers (Herbivores):
Primary consumers, also known as herbivores, are organisms that directly feed on producers. They obtain energy by consuming plant material. Examples of primary consumers include rabbits, deer, and cows.

3. Secondary Consumers (Carnivores):
Secondary consumers are organisms that consume primary consumers. They are carnivores that obtain energy by feeding on herbivores. Examples of secondary consumers include snakes, wolves, and lions.

4. Tertiary Consumers (Top Carnivores):
Tertiary consumers occupy the highest trophic level in a food chain. They are top carnivores that feed on other carnivores. Examples of tertiary consumers include eagles, sharks, and tigers.

5. Decomposers:
Decomposers play a crucial role in the food chain by breaking down dead organisms and organic waste into simpler substances. They include bacteria, fungi, and detritivores. Decomposers release nutrients back into the ecosystem, allowing the cycle to start again.

In summary, the correct sequence of a food chain starts with producers (autotrophs), followed by primary consumers (herbivores), then secondary consumers (carnivores), and finally tertiary consumers (top carnivores). Decomposers play a vital role in recycling nutrients and completing the food chain.

It's important to note that food chains are interconnected, and organisms can occupy multiple trophic levels depending on their diet. Additionally, some organisms can occupy more than one trophic level simultaneously, making food webs a more realistic representation of energy flow in ecosystems.