The answer is C, serve a different function.
In multicellular organisms, same types of cells makes up a tissue, and tissue makes up organs, then eventually organs work together as an organ system.
For example in humans, stomach cells makes up the stomach tissues, and stomach tissues makes up the stomach. stomach can help digest food and mix food molecules together. But in opposition, red blood cells is present in the blood tissue, however, their function is not to digest food, but is to transport oxygen.
Therefore, we can see that different cells serve a different function.
