A new organism that is composed of many cells, gets its nutrition from decaying organisms belongs to Kingdom Fungi
Step-by-step explanation:
The organism which is described in the question states that the organism is multicellular in nature and is composed of cell wall and its mode of nutrition is saprophytic that is it obtains its nutrients by feeding on dead and decayed organisms.
From this information it can be inferred that the organism would belong to kingdom fungi as the members of this kingdom are characterized by its unique mode of nutrition which is called saprophytic.