What is the difference between a unicellular organism and multicellular organism?

Select one:
a. Unicellular organisms are made up of a single cell, while multicellular organisms are made up of several cells.
b. Unicellular organisms are not living, while multicellular organisms are living.
c. Unicellular organisms are highly organized into tissues and organs, while multicellular organisms are simple.
d. Unicellular organisms do not need water, while multicellular organisms do need water.