Which of the following is a producer?

The correct answer is: Algae

Fungi plays the role of decomposer in an ecosystem, breaking down plant and animal debris, thereby releasing nutrients in the soil.
Herbivores directly feed upon the producers and do not eat anything other than vegetation.
Protists are mainly decomposers while some species such as algae may act as producers.
Algae are able to use sunlight to produce their own food. Hence, algae is a producer.