Final answer:
The outer layer of a cell membrane contains the hydrophilic phosphate group heads of phospholipid molecules, which face the aqueous environments outside and inside the cell.
Step-by-step explanation:
Cell membranes are made up of a phospholipid bilayer, with each phospholipid molecule consisting of a hydrophilic head and two hydrophobic fatty acid tails. The hydrophilic heads face the aqueous environments on both the outside and the inside of the cell because they are attracted to water. On the contrary, the hydrophobic tails, which comprise the fatty acid chains, face each other in the middle of the cell membrane to avoid water. Therefore, the correct answer to the question is c. phosphate groups; hydrophilic. Phospholipids are critical components of the plasma membrane of cells, which allows cells to maintain their integrity in a water-based environment.