The cell membrane functions most like a(n) A.) security guard
B.) brick wall
C.) painter
D.) delivery truck

It's A, a security guard

Explanation:

The cell membrane is like a security guard because it controls what goes in and out of the cell.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.