Lichens are:

Symbiosis is a close, long-term biological interaction between two different organisms living together, most often to the mutual benefit of both. Lichens are the classic textbook example of a mutualistic symbiotic association: a composite organism formed by a fungus (the mycobiont) together with a photosynthetic partner, either green algae or cyanobacteria (the photobiont), living as a single functional unit.

In this partnership, the fungus builds a protective structure that retains moisture and shields the photosynthetic partner from UV radiation and drying out, while the alga or cyanobacterium uses photosynthesis to manufacture carbohydrates that nourish both partners. This partnership lets the composite organism survive in habitats (bare rock, tree bark, exposed soil) that would be far harder for either partner to persist in alone, which is exactly why this composite organism is classified as symbionts rather than as any single organism type.

Contrast with the other options, by value:

• Parasites — live at the expense of a host and harm it; here, the photosynthetic partner is not harmed, so this one-sided, harmful relationship does not fit.

• Chemoautotrophs — organisms that derive energy by oxidizing inorganic chemical compounds rather than using sunlight; the photosynthetic partner here relies on ordinary photosynthesis, not chemical oxidation.

• Decomposers — obtain nutrition by breaking down dead organic matter; this composite organism instead manufactures its own food through photosynthesis and grows on bare rock, bark, or soil rather than feeding on decaying material.

Therefore, lichens are correctly classified as symbionts — a mutualistic symbiotic partnership between a fungus and a photosynthetic partner.