An organism made up of many cells is called a

Concept: All of the living beings are made up of either one or numerous cells. Organisms which have more than one cell are known as multicellular. 

A life form which comprises of single cell as entire body are known as unicellular living being or single-celled organism. The nonappearance of cells is known as acellular. The multicellular organisms are composed of more than one cell, with groups of cells differentiating to play other specialized functions. In humans, the cells differentiate early in development to become nerve cells, skin cells, muscle cells, blood cells, and other types of cells.

Therefore, such organisms are made up of many cells and are called multicellular organisms.

Hence, the correct answer is option B.