Final answer:
Cholesterol is found within the hydrophobic core of plasma membranes, among the phospholipid tails, due to its amphiphilic structure with a polar head and a nonpolar tail.
Step-by-step explanation:
Cholesterol is an integral part of plasma membranes and is amphiphilic in nature, meaning it has both a hydrophilic (polar) and a hydrophobic (non-polar) part. The cholesterol molecule contains an -OH group as its polar head and a hydrophobic cholestane structure as its tail. Considering the structure of a plasma membrane, where phospholipids form a bilayer with their hydrophobic tails facing inside and hydrophilic heads facing the aqueous environment, cholesterol fits within the membrane by intercalating among the phospholipid tails in the hydrophobic core of the membrane. Thus, the most accurate answer is that cholesterol is found within the tail bilayer, effectively making the correct choice from the provided options 'c. within the tail bilayer'.