Multicellular organisms arose because a a part could die and be removed b they could eat unicellar organisms or c unicellar organisms began to die offwith the oxygen filled atmosphere

b they could eat unicellar organisms

Multicellular organisms arose because of the process called endosymbiosis. The endosymbiosis theory posits that other unicellular organisms ate other organisms which has benefitted them into processing new mechanisms and functions.

A Eukaryotic cell contains a nucleus; more than one chromosome and is typically a multi-celled organism.  Both plant and animal cells are eukaryotic cells.

The answer is a i believe