Which statement is an example of a scientific theory?

a.
all living organisms are composed of one or more cells, which came from pre-existing cells.
b.
hummingbirds do not like the color white because they visit other colored flowers more often.
c.
the amount that caffeine affects a person's heart rate differs based on the person's age.
d.
the earth is the third planet from the sun and has one moon, which orbits the earth.