Which of the following has the largest molecules: starch, iodine, or water?

Starch has the largest molecule out of all of starch, iodine and water. This is because it is composed of numerous units of glucose, so it's technically a polymer.

Glucose is small enough to pass through the wall of the gut, but starch has to be broken down into glucose during digestion. That way, glucose molecules can be transported easily throughout the body in the blood stream before being used for respiration.

Starch will appear anywhere from yellow/brown to blue/black when iodine is applied to it. A light beam will be visibly dispersed when shined on starch; an effect that is called the Tyndall Beam effect.