A fungus and an alga coexist in a symbiotic partnership to create the complex living form known as ________. 

Some organisms coexist and share resources like food and shelter such a relationship is called a symbiotic relationship.

An example of obligate symbiosis is the relationship between algae and fungi in lichen. Alga is the source of food as it can synthesize food and the fungus absorbs water and minerals for the algae.

Hence, the correct answer is lichens.