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.

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.

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.

Explanation: The acid present in the human stomach is hydrochloric acid (HCl). It helps in the digestion of food by breaking down proteins and activating the enzyme pepsin. HCl also kills harmful microorganisms present in the food, aiding in maintaining a healthy digestive system.

Explanation: Amniocentesis is a medical diagnostic procedure used to determine the genetic and chromosomal makeup of a developing fetus. It involves the withdrawal of a small amount of amniotic fluid from the uterus using a fine needle. This fluid contains fetal cells that can be tested to identify genetic disorders such as Down’s syndrome and also to determine the sex of the fetus. However, using amniocentesis for sex determination is illegal in India under the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act to prevent female foeticide.

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: The human retina contains two types of photoreceptor cells — rods and cones. Cones are concentrated in the central part of the retina (fovea) and are sensitive to different wavelengths of light — red, green, and blue. These enable colour vision and visual acuity in bright light. Rods, on the other hand, function in dim light and provide black-and-white vision.

Explanation: Fish respire through specialized organs called gills, located on either side of their head. Gills contain thin filaments with a large surface area and rich blood supply that allow exchange of gases. When water passes over the gill membranes, oxygen diffuses into the blood and carbon dioxide diffuses out. This process enables aquatic respiration and supports survival underwater.

William Harvey, an English physician, discovered the process of blood circulation in 1628. He demonstrated that the heart acts as a pump to circulate blood continuously throughout the body via a closed system of arteries and veins. His work, published in "De Motu Cordis" (On the Motion of the Heart and Blood), laid the foundation for modern cardiovascular physiology.

Other options (for context)

• Mary Anderson: Invented the windshield wiper, not related to medicine.
• Virginia Apgar: Developed the Apgar score for assessing newborn health.
• Robert Feulgen: Discovered a staining technique for DNA, not blood circulation.

Explanation: Entomology is a branch of Zoology that focuses on the study of insects, including their structure, classification, behavior, ecology, and evolution. Insects make up more than 75% of all known animal species on Earth and play crucial roles in pollination, decomposition, and food chains. Scientists who study insects are known as entomologists.