Explanation:

Cnidarians: Many cnidarians (like corals) establish symbiotic relationships with algae (such as zooxanthellae). The algae live inside the corals and provide them with food through photosynthesis, while the corals provide the algae with protection and access to light.

Fungi: Many fungi establish symbiotic relationships with plants, known as mycorrhizae. The fungi assist the plants by extending the root's nutrient absorption capability, and in return, the plant provides the fungi with sugars produced during photosynthesis. Lichens are another example where fungi live symbiotically with algae or cyanobacteria.

Protozoa: While some protozoa can establish mutualistic relationships (for example, certain termite gut protozoa that help in digesting cellulose), they are not as universally recognized for their symbiotic relationships as cnidarians and fungi.

Hence, Cnidarians and Fungi are the groups that are well known for establishing symbiotic relationships with other organisms.