Assertion: Lichens are formed by symbiotic associations of autotrophs and heterotrophs.
Reason: A lichen has an alga as its autotrophic component and fungus as heterotrophic component which show mutual benefit. 

Assertion (A) is correct. Lichens are indeed the result of a mutualistic symbiotic relationship between a photosynthetic partner and a fungal partner. The photosynthetic partner provides organic compounds through photosynthesis, while the fungal partner provides a protective structure and absorbs water and nutrients.

Reason (R) is also correct. Lichens often include an alga as the photosynthetic partner, they can also include cyanobacteria. The fungus is the heterotrophic component. In reality, both the alga (or cyanobacterium) and the fungus in lichens benefit from the association, and they engage in mutualistic interactions. 

Both assertion (A) and reason (R) are correct and reason explains the assertion