Biology

The correct answer is:

Centuries of observations and experiments verify the idea.

Here's why:

Cell theory is considered a theory because it is well-supported by extensive evidence from a wide range of scientific observations and experiments. Over centuries, scientists have consistently found that all living organisms are composed of cells and that the cell is the basic unit of life. This idea has been repeatedly tested and confirmed in countless experiments across different fields of biology, from microscopy to genetics.

Why the other options are not correct:

A."Since we haven't checked every organism for cells, we cannot call it a hypothesis."

This is incorrect. While it's true that we can't check every single organism, the fundamental concepts of cell theory have been verified in such a large variety of organisms (including all known forms of life) that the theory is widely accepted as accurate. It's not about checking every organism, but about the overwhelming body of evidence across many species and domains of life.