Which of the following is NOT a major class of laser?

The correct answer is the choice relating to liquid-hydrogen. Among the primary classes of lasers, solid-state (crystal), semiconductor, and gas lasers are all widely recognized categories that reflect the material or medium the lasers use to generate light.

Solid-state lasers utilize solid materials like crystals or glass doped with ions, which allow for efficient light emission. Semiconductor lasers, often used in various electronics, operate using semiconductor materials to produce laser light. Gas lasers, on the other hand, use a gas as the lasing medium, which is common in applications such as cutting and medical procedures.

Liquid-hydrogen does not represent a major class of laser technology. While there are experimental lasers that may involve liquid hydrogen, it does not have the same established framework or practical applications as the other mentioned classes. Thus, identifying liquid-hydrogen as not belonging to the major classes of lasers is accurate in this context.