Answer:
The correct answer is a. the photosynthetic part of a lichen.
Step-by-step explanation:
Lichen is a symbiotic association in which the algae and fungi are associated with each other. Fungus can not make there own food so they are not photosynthetic like algae or plants and in lichens algae are the photosynthetic part while fungus plays the role of absorbing nutrients for the algal partner.
Lichens are found all over the world and they absorb industrial pollution very quickly so are also referred to as pollution indicators of the environment as they do not grow in a polluted area.
Therefore the photosynthetic part of lichen is algae, not fungus so the correct answer is a.