Which of the following units of measurement is used to measure very short wavelengths of light?

The correct choice is indicated because both the angstrom and millimicron are units specifically designed for measuring very short wavelengths of light, particularly in the range applicable to ultraviolet and visible light. An angstrom is equal to 10^-10 meters, making it a fitting unit for expressing atomic and molecular scales of measurement, which often pertain to light wavelengths.

On the other hand, a millimicron, also known as a nanometer, is equivalent to 10^-9 meters. While it is often used interchangeably with nanometers in many contexts, it serves the same purpose in measuring very short wavelengths of light.

Both units are integral within the field of optics and atomic physics, as they allow scientists to describe and work with the fine details of light phenomena that occur on such small scales. Given that both units are appropriate for the task, it stands to reason that the best answer includes them both.