Angstrom measures wavelength; option [B]. It is a unit of length used predominantly in fields such as atomic physics, spectroscopy, and crystallography. Named after the Swedish physicist Anders Jonas Ångström, 1 Angstrom is equivalent to 0.1 nanometers or 10^(-10)meters. This unit is particularly useful in expressing the sizes of atoms and molecules, as well as the wavelengths of electromagnetic radiation, including light. In spectroscopy, for instance, Angstroms are commonly used to measure the wavelengths of spectral lines emitted or absorbed by atoms and molecules. This allows scientists to analyze the composition and properties of substances based on the characteristic wavelengths of light they emit or absorb. Therefore, Angstrom serves as a fundamental unit for quantifying the spatial periodicity of waves and the distances between atomic or molecular structures. Consequently, the correct option is [B] Wavelength, as Angstroms are primarily utilized to measure the lengths of waves, whether they are electromagnetic waves or the characteristic sizes of atomic and molecular structures.