Agroup of tissues working together to perform a closely related function is called a(n)
a. organ system.
b. organ.
c. organism.
d. cell.

D is incorrect. A cell is the most basic unit of life.
C is incorrect. An organism is a living thing (e.g. human, animal)
A is incorrect. An organ system is many organs together (e.g. digestive system, respiratory system)

The correct answer is B. Many tissues forming together forms an organ (e.g. heart, lung).

Hope this helps!!