When there is not sufficient carbohydrate to completely metabolize fatty acids, these molecules form: ?

The right answer is ketone bodies (or ketones).

Ketones refer to certain substances produced during the breakdown of fats in the body.
Much of the acetyl CoA produced by the degradation of fatty acids and glucose is oxidized in the Krebs cycle. However, a fraction of this acetyl CoA is converted in the liver cells into ketone bodies (acetoacetate, D-β-hydroxybutyrate, and acetone).
Ketones are usually eliminated by the kidneys in the urine. But if they are produced in excess and in too high concentration, they become toxic. This is known as ketoacidosis, which usually manifests as a complication of diabetes.