Explanation: The troposphere is the lowest layer of the Earth's atmosphere and is closest to the Earth's surface. It extends up to about 8–15 km from the surface, depending on latitude and season. This layer contains about 75% of the total atmospheric mass and almost all the water vapor and weather phenomena such as clouds, rain, and storms occur here. The temperature decreases with increasing altitude in this layer. The boundary between the troposphere and the stratosphere is known as the tropopause.

The Red Data Book contains information about endangered and threatened species of plants and animals. It is published by the International Union for Conservation of Nature (IUCN). The book categorizes species based on their risk of extinction, such as Critically Endangered, Endangered, Vulnerable, and Extinct. It serves as a global reference for monitoring biodiversity conservation and guiding environmental protection policies.

Other options (for context)

• All plant species: Incorrect — it includes only those plants that are endangered or threatened.
• All animal species: Incorrect — it covers only the endangered and vulnerable ones, not all animals.
• All extinct species: Incorrect — it includes species at risk of extinction, not only those already extinct.

Explanation: Sodium (Na⁺) plays a crucial role in the transmission of nerve impulses in the human body. Nerve cells (neurons) transmit signals through an electrochemical process known as the nerve impulse or action potential. During this process, the movement of sodium (Na⁺) and potassium (K⁺) ions across the nerve cell membrane generates an electrical charge. The rapid influx of sodium ions causes depolarization, which allows the nerve impulse to travel along the neuron, ensuring proper communication between the brain, spinal cord, and body.

Explanation: Windows XP is an operating system (OS) developed by Microsoft. An operating system is system software that manages computer hardware, software resources, and provides services for running application programs. Windows XP, released in 2001, was one of the most popular OS versions used for both personal and business computers. It provides a graphical user interface (GUI), multitasking, and file management capabilities.

Explanation: The ATM (Automated Teller Machine) was invented by John Shepherd-Barron in 1967. He designed the first cash dispensing machine, which was installed by Barclays Bank in London, UK. The first ATM used special paper vouchers impregnated with a radioactive compound instead of plastic cards. The idea revolutionized the banking sector by allowing people to withdraw money anytime without visiting a bank counter. Modern ATMs now use magnetic or chip-based cards, PIN authentication, and networked systems for secure transactions.

Explanation: The burning of a candle is a chemical change because new substances such as carbon dioxide, water vapour, and heat energy are produced during the reaction between wax and oxygen. This change is irreversible and results in a permanent alteration of the chemical composition of the material. In contrast, melting of wax and vapourisation of water are physical changes since no new substances are formed and the process can be reversed by cooling.

Graphite allows electric current to pass through it. Although it is a form of carbon, its atoms are arranged in layers with free-moving electrons, making it a good conductor of electricity. This property makes graphite useful in electrodes, batteries, and brushes of electric motors.

• Glass, Rubber, PVC: All are insulators and do not conduct electricity.

Xylem is a type of vascular tissue in plants responsible for the transportation of water and dissolved minerals from the roots to the stems and leaves. The upward movement of water through xylem vessels occurs due to processes like transpiration pull, root pressure, and capillary action. Xylem also provides mechanical support to the plant.

Other options (for context)

• Food: Transported by phloem, not xylem.
• Nutrient: Minerals are carried along with water but the primary function of xylem is water transport.
• Both food and water: Incorrect — only phloem transports food; xylem transports water.