There exists a close association between the alga and the fungus within a lichen. The fungus :

Lichens are symbiotic associations between fungi and algae (or cyanobacteria). The fungus in a lichen provides several benefits to the alga, including protection from extreme temperatures and desiccation, anchorage to the substrate, and absorption of water and minerals from the environment. In return, the alga photosynthesizes and provides nutrients to the fungus. While the alga does fix CO2 through photosynthesis, the fungus does not fix nitrogen. The correct answer is option (B) provides protection, anchorage and absorption for the alga. Think of it this way: the fungus in a lichen plays a critical role in supporting the alga by providing it with a safe and stable environment, allowing the alga to carry out photosynthesis and produce nutrients for both partners.