The correct answer is option 1, 1 A. X-rays have a very short wavelength, much smaller than visible light. The wavelength of X-rays is typically measured in angstroms (A), which is equal to 10^-10 meters.

Option 2, 1 μm (micrometer), is incorrect because it is larger than the correct answer. 1 μm is equal to 10^-6 meters, which is much larger than the angstrom unit used to measure X-ray wavelengths.

Option 3, 1 mm (millimeter), is also incorrect because it is much larger than the correct answer. 1 mm is equal to 10^-3 meters, which is significantly larger than the wavelength of X-rays.

Option 4, 1 cm (centimeter), is also incorrect because it is even larger than the previous options. 1 cm is equal to 10^-2 meters, which is much larger than the wavelength of X-rays.

Therefore, the correct answer is option 1, 1 A, as X-rays have a very short wavelength on the order of angstroms.