Some complex multicellular organisms are organized at four levels to increase efficiency. what is the correct order of organization, from the simplest to the most complex?
a. organ, tissue, cell, organ system
b. cell, organ, organ system, tissue
c. tissue, cell, organ system, organ
d. cell, tissue, organ, organ system

The answer is letter D. The simplest to most complex life organization is: cell, tissue, organ, organ system. Multicellular organisms are composed of the building blocks of life called cells. When cells are grouped together they form tissues that perform specific functions in an organism. Further in the line, when tissues are grouped together they form organs; organs are responsible in performing bodily functions in organisms. The heart, lungs, and brain are the most important organs in the human body. When organs work in an interconnected way they are called organ systems.