Neon is commonly used in which of the following applications?

Neon is most famously known for its use in red neon signs and lasers, making this the correct choice. Neon gas emits a bright red-orange glow when an electric current passes through it, a property that is utilized in various forms of signage and lighting, giving them an eye-catching appeal. The characteristic glow arises from the excitation of neon atoms, which release energy in the form of light as they return to their ground state.

Other choices relate to applications using different gases or technologies. While refrigeration units generally rely on specific refrigerants like fluorocarbons or ammonia, electric bulbs primarily use gases like argon or krypton to prevent the filament from burning out quickly. Medical diagnostics may involve various gases or compounds, but neon is not typically utilized in this context. Thus, the use of neon in signs and lasers stands out as its most recognizable application.