Explanation

A lichen is a symbiotic association between a fungus (the mycobiont) and a photosynthetic partner — an alga or a cyanobacterium (the photobiont). It is therefore a composite 'organism' made of two (sometimes more) partners, so 'single organism' is wrong. Bacteria can occur as secondary associates but are not the defining photosynthetic partner, and a lichen is not composed only of fungi. Hence B is the correct description.