What does the "theory" mean in "Cell Theory?"

a. That there are a tremendous amounts of experiments supporting and broad scientific agreement on a subject.
b. That it is just an idea that could be easily disproven.
c. That is is something being actively tested to find out if a hypothesis is correct.
d. That it is an irrefutable fact and is accepted as a scientific law.

b. That it is just an idea that could be easily disproven.