The correct answer is option 4: 1 angstrom. X-rays have a very short wavelength, and it is measured in angstroms. An angstrom is a unit of length equal to 10^-10 meters. X-rays have wavelengths in the range of 0.01 to 10 angstroms.

Option 1: 1 mm - This option is incorrect. X-rays have much shorter wavelengths than 1 mm. In fact, 1 mm is about 10 million times longer than the wavelength of X-rays.

Option 2: 1 cm - This option is incorrect. Similarly to option 1, 1 cm is much larger than the wavelength of X-rays.

Option 3: 1 micron - This option is also incorrect. While 1 micron is smaller than both 1 mm and 1 cm, it is still much larger than the wavelength of X-rays. 1 micron is equal to 10^-6 meters, which is still much larger than 1 angstrom.

Therefore, the correct option is 1 angstrom, as X-rays have wavelengths on the order of magnitude of 1 angstrom.