Final answer:
The physical relationship of the partners involved in lichens is mutualistic.
Step-by-step explanation:
The physical relationship between the partners involved in lichens is mutualistic. Lichens are formed by a symbiotic relationship between a fungus (usually a member of the Ascomycota or Basidiomycota) and a photosynthetic organism (a eukaryotic alga or a prokaryotic cyanobacterium).
The fungus provides protection from dryness and excessive light by encasing the algae in its mycelium and also attaches the lichen to its substrate. In return, the photosynthetic organism provides carbon and energy in the form of carbohydrates, and some cyanobacteria additionally fix nitrogen from the atmosphere, contributing nitrogenous compounds to the association.
Overall, the relationship between lichens and their partners falls within the continuum between conflict and cooperation, with the fungus benefiting more than the photosynthetic organism. Therefore, the best description of the physical relationship of the partners in lichens is mutualistic.