Final answer:
Organisms that are mostly multicellular, have chitin cell walls, and are heterotrophic belong to the fungal kingdom. Unlike plants, fungi do not photosynthesize and have unique cell wall components. They include a diverse range from unicellular yeasts to multicellular mushrooms.
Step-by-step explanation:
A group of organisms that are mostly multicellular, with a few unicellular examples, possess a cell wall made of chitin, and are heterotrophic, can be classified as Fungi. Fungi are unique among eukaryotic organisms, as they do not photosynthesize like plants. Unlike plants which have cellulose in their cell walls, fungi have cell walls composed of chitin, which is also found in the exoskeletons of arthropods. Fungi can be unicellular like yeasts or form more complex multicellular structures such as mushrooms. They play a critical role in ecosystems as decomposers, breaking down organic materials and recycling nutrients.