The correct option is D both a and b
Symbiosis is a long-term, intimate and specific association between organisms of two or more species. Here there is mutual give and take relationship between the organisms involved. Algae occur in association with fungi to form lichens where algae provide food to lichens and lichens provide protection and nutrition to the algae.
Algae occurs in a symbiotic relationship with animals like the sloth. Sloth is a slow-moving animal. The algae camouflages the sloth and protects it from predators. Sloth, in turn, provides protection and water to the algae.
Fungi occur in a symbiotic relationship with plant roots of some gymnosperms and such roots are called mycorrhizae.