The correct answer is the organ.
The multicellular species are formed of various parts, which are required for survival. These parts are differentiated into the levels of the organization. There are five different levels, which are named as cells, tissue, organs, organ systems, and organisms.
When two or more layers of tissue function together, they produce an organ. All the animals comprise vital organs, without which they cannot survive. These include the kidneys, liver, lungs, brain, and heart.