What prevents ions from crossing the plasma membrane via
simple diffusion?
a. ions are too large.
b. ions carry a charge.
c. ions are hydrophobic.
d. ions do not produce a concentration difference.
e. ions cannot interact with membrane-bound
proteins.

Simple diffusion is the concept of a substance moving from an area of high concentration to a lower concentration without energy input. Because the plasma membrane is made of lipids which are non-polar, this membrane readily allows non-polar substances to diffuse through easily. If a substance has a charge like that of an ion, it will be difficult for the ion to cross the plasma membrane through simple diffusion. The answer should be B. 

Ithink you should go back and learn in school rather then here asking questions