The option that best explains how an underground portion of a plant, such as a potato, can contain starch is:
"Some producers perform photosynthesis in their leaves and store extra glucose (in the form of starch) in their roots."
This statement accurately reflects the process by which plants absorb sunlight through their leaves to produce glucose via photosynthesis. The excess glucose is then converted into starch and stored in various parts of the plant, including underground structures like tubers (e.g., potatoes) for later use as energy.