Common gas lasers include helium-neon, argon-ion and neon variants, so the best choice is all of these.

Wien's displacement constant is approximately 2.9 × 10^-3 m·K in SI units.

The hardness (penetrating power) of X-rays depends on the accelerating voltage; a larger potential difference produces higher-energy (harder) X-rays.

A spectrometer analyzes light by wavelength or frequency and measures spectral properties over portions of the EM spectrum.

Raising the accelerating voltage increases the kinetic energy of electrons and thus the maximum photon energy (shortest wavelength) produced.

The ozone layer (here named 'ozonosphere') absorbs much of the Sun's ultraviolet radiation.

CAT stands for Computerized Axial Tomography in medical X-ray technology, as indicated by the OCR answer hint.

A wavelength of 10 cm (0.1 m) is in the microwave region of the electromagnetic spectrum.

Kinetic energy K = hν - φ, so K(2ν) = 2hν - φ = 2K(ν) + φ, and since φ>0 this is greater than twice the original K.

Kirchhoff's laws explain how absorption lines (Fraunhofer lines) form when cooler gas absorbs specific wavelengths from a continuous source behind it.