Final answer:
Option (b), The hydrophobic portion of the plasma membrane is formed by the fatty acid tails of phospholipids, which arrange themselves inside the bilayer away from water.
Step-by-step explanation:
The hydrophobic portion of the plasma membrane is formed by the fatty acid tails of phospholipids. Phospholipids are a major component of the plasma membrane, and each phospholipid molecule features a hydrophilic head, which includes a phosphate group, and a hydrophobic tail comprised of two fatty acids.
In the plasma membrane, phospholipids arrange themselves into a bilayer, with the hydrophobic tails facing inward, away from water, and the hydrophilic heads facing outward towards the aqueous environment.
The correct answer to the question 'Which of the following forms the hydrophobic portion of the plasma membrane?' is option b, fatty acid tails.