A lichen is an organism made up of an alga and a fungus. The alga supplies energy for itself and the fungus through photosynthesis. The fungus provides a protected space for the alga to live. This is an example of .

Mutualism is a relationship in which both organisms benefit. Here, the alga makes food through photosynthesis for itself and the fungus, while the fungus provides the alga with a protected place to live. So this is mutualism, choice A.

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