How do you know that's a living this is multicellular

Multicellular organisms are organisms that consist of more than one cell, in contrast to unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni - and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

Explanation: Google