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!!

Mg is used to measure mass