What is the smallest unit of a living organism?

Answer: a cell

Explanation: a cell is the basics unit of life. It can also be said to be the building block of life. All lifes from the simple procaryotic bacteria to the more specialised eucaryotic humans are made of cells.

Several cells of the same function form tissues. Several tissues of same function form organs. Different organs of the same function form a system. Several system aggregate to form an organism.