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

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. 

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.