Explanation: Sound waves are not electromagnetic waves; they are mechanical waves that require a material medium (like air, water, or solids) for propagation. In contrast, radio waves, X-rays, and visible light are electromagnetic waves that can travel through a vacuum.

The full form of LAN is Local Area Network. It is a network that connects computers and devices within a limited geographical area such as a home, office, school, or building. LANs enable data sharing, resource sharing (like printers or internet connections), and communication between connected devices. Common technologies used in LANs include Ethernet and Wi-Fi.

Other options (for context)

• Line Area Network: Incorrect — not a recognized networking term.
• Linear Area Network: Incorrect — there is no such standard network type.
• Land Area Network: Incorrect — misinterpretation of LAN abbreviation.

Explanation: Unwanted or unsolicited e-mails sent in bulk are called spam. These messages often contain advertisements, phishing links, or malware. They clutter inboxes and can pose security risks. The term “spam” originated from a comedy sketch by Monty Python, where the word was repeatedly used, much like unwanted messages.

Explanation: The branch of science that deals with the study of the production, behavior, and effects of materials at very low temperatures is called Cryogenics. The word comes from the Greek words “kryos” meaning cold and “gen” meaning to produce. Cryogenics involves cooling substances to temperatures below −150°C (123 K). It has applications in liquefying gases (like oxygen and nitrogen), cryopreservation of biological samples, superconductivity research, and rocket fuel storage (e.g., liquid hydrogen and oxygen).

Explanation: The element hydrogen is common to all acids. Acids are substances that produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) when dissolved in water. This property gives acids their characteristic sour taste, the ability to turn blue litmus paper red, and to react with bases to form salts and water. Examples include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO₃)—all of which release hydrogen ions in aqueous solution.

Explanation: The transmission of light through optical fibres is based on the principle of Total Internal Reflection (TIR). When light travels from a denser medium (core of the fibre) to a rarer medium (cladding) and strikes the interface at an angle greater than the critical angle, it gets completely reflected back into the core instead of refracting out. This continuous internal reflection allows light to travel long distances through the fibre with minimal loss, even when the fibre is bent. Optical fibres are widely used in telecommunication, internet data transfer, and medical endoscopy.