Final answer:
The relationship between algae and fungi in a lichen is mutually beneficial, categorized as symbiosis. Algae provide nutrients via photosynthesis, and fungi offer structure and shelter, along with aiding in water and nutrient absorption.
Step-by-step explanation:
The symbiotic relationship between algae and fungi in a lichen is considered mutualistic, which means both organisms benefit from living together. This type of relationship can be classified as symbiosis (c), where each organism contributes something to the other. The algae, through the process of photosynthesis, provides nutrients (a) in the form of energy-rich food molecules to the partnership. The fungi, on the other hand, contributes by providing structure and shelter (b), ensuring protection from environmental stresses such as dryness and excessive light, which allows the algae to survive in a wider range of habitats than it could on its own. Additionally, the fungi assist with the absorption of water and soluble nutrients from the environment, which supports the algae's growth and health.