Question

In a multicellular organism, organs carry out a variety of life functions. In a single-celled organism, these functions are performed by

A.tissues
B. organelles
C. organ systems
D. organs

Answer 1

B. organelles because organelles are structures in the cell for example the plasma membrane, nucleus, mitochondria, etc. which they all have their own functions to make life possible. 

Answer 2

The correct option is B.
A multi cellular organisms has many cells which carry out various function and which are organised into tissues, organs and organ system. A unicellular organism on the other hand has only one cell with different cell organelles. Thus, in unicellular organisms, it is the cell organelles that carry out different functions.