The basic unit of all forms of life is a(n)
a. organ system.
b. organ.
c. tissue.
d. cell.

The basic unit of life is a D. cell. 
D is the correct answer because although life is also made up of the other options, in the end, they are all made up of cells. A tissue is a group of specialized cells, and an organ is made up of different tissues. An organ system is a system of organs that work together.
Hope that helped you.

The correct answer is c