Answer
The membrane around an oil droplet could be a monolayer, since the fatty acids could interact with the hydrophobic oils inside the droplet and thus not need the second layer of a bilayer for stability.
Work Step by Step
The key here is to see that a membrane bilayer has two layers so that they hydrophobic fatty acid tails are stabilized by interacting with one another, but that is is not needed if the material interior to the membrane is a lipid droplet.
