Algae at the beginning of a food chain would be considered b) primary producers.
Explanation: In the ecosystem, primary producers are organisms that can produce their own food through photosynthesis or chemosynthesis. Algae, which are primarily photosynthetic organisms, convert sunlight into energy, forming the base of the food web. They utilize carbon dioxide and water, and through the process of photosynthesis, they create organic compounds that serve as food for various primary consumers. Unlike heterotrophs, which depend on other organisms for energy, or decomposers, which break down dead organic matter, primary producers like algae are essential for establishing energy flow in an ecosystem.