The correct option is D Both a and b
In unicellular organisms, the cell is considered to be equivalent to an organism. The single cell performs all functions of life.
Multicellular organisms usually have cells (cell aggregate- Porifera), tissues (Cnidaria, Ctenophora), organs (Platyhelminthes) and organ systems (Aschelminthes to Mammalia). There are different levels of organization in different organisms which perform various functions.