Wavelengths of x-rays are measured in:

X-rays are a form of electromagnetic radiation, and their wavelengths are typically measured in angstroms. An angstrom is defined as (1 \times 10^{-10}) meters, which is a convenient unit for expressing the very small wavelengths of x-rays, typically ranging from about 0.01 to 10 nanometers (or 0.1 to 100 angstroms). This measurement scale is helpful when discussing wavelengths on the atomic and subatomic levels, where x-rays interact with electron shells and the overall structure of atoms.

Other units like millimeters and centimeters are significantly larger than the wavelengths involved in x-ray measurements, making them unsuitable for this purpose. The roentgen is a unit of measurement related to exposure to ionizing radiation, specifically the amount of radiation needed to produce a certain ionization in air, rather than a direct measure of wavelength. Therefore, when discussing the wavelengths of x-rays, angstroms are the appropriate and precise unit of measurement.