What type of relationship does the lichen represent here?
A lichen is a combination of fungus and algae that lives on the sides of trees, rocks, and other materials. The fungus provides the algae with water and minerals and the algae uses the water and minerals to make food for both organisms. 

This is mutualism because the fungus and the algae both benefit from each other. It becomes parasitism when only one of the living things is benefiting from the relationship. An example of this is the head lice in humans.

The head lice can suck the blood of its human host but the human will not get anything good from the head lice. It cannot be commensalism too because, in this type of relationship, one benefits while the other living thing is not affected. For example, the orchids that may grow on tree branches. Orchids will benefit from the branches but the branches will not be affected by the orchids at all.