Infrared signals can be used for short range communication in a closed area…

Answer: line of sight (Infrared)

Why: Infrared signals are in the optical region of the spectrum and propagate primarily in straight lines. They do not penetrate walls or rely on ionospheric or ground-wave mechanisms, so communication is limited to short ranges where the transmitter and receiver have a clear path or where limited reflections inside the room can be used.

• Practical implication: Devices using infrared (for example, remote controls or short-range data links) typically require line-of-sight or operate over short distances indoors.

• Contrast with other propagation types: ground, sky (ionospheric), and space propagation describe radio-wave behaviours over longer distances or through different media and are not suitable descriptions for short-range infrared communication.