Mushrooms belong to the group of organisms known as fungi. Fungi are a diverse group of organisms that obtain their nutrients by decomposing organic matter or by forming mutualistic relationships with other organisms. Unlike plants, fungi do not have chlorophyll and cannot produce their own food through photosynthesis. Instead, they obtain nutrients by breaking down dead organic material or by living in symbiotic associations with other organisms.

Option 1, Algae, refers to a group of photosynthetic organisms that can be found in various aquatic habitats. These organisms are capable of producing their own food through photosynthesis and are not closely related to fungi.

Option 2, Ferns, are a type of vascular plant that reproduces through spores. While both fungi and ferns reproduce through spores, they belong to different kingdoms and are not closely related.

Option 4, Lichens, are unique organisms formed by a symbiotic relationship between a fungus and a photosynthetic partner, usually algae or cyanobacteria. Although mushrooms and lichens are both fungi, mushrooms are not a type of lichen.

In conclusion, mushrooms belong to the group of organisms known as fungi.