Explanation: Shadows are formed when an opaque object comes in the path of light. Opaque objects do not allow light to pass through them, causing a dark region called a shadow to form on the opposite side. Transparent and translucent objects allow light to pass fully or partially, so they form faint or no shadows.

Explanation: SONAR (Sound Navigation and Ranging) is a system used in ships and submarines to determine the depth of the ocean or to locate underwater objects. It works by sending sound waves into the water and measuring the time taken for the echo to return after reflecting from the seabed or any object.

Explanation: Gamma rays are the most dangerous type of radiation because they have very high energy and strong penetrating power. They can pass through most materials, including human tissue, and may cause serious cellular and genetic damage. Proper shielding with dense materials like lead is required for protection.

Explanation: A carnivore is an animal that primarily eats the flesh of other animals. The tiger (Panthera tigris) is a true carnivore and an apex predator in its ecosystem. It has sharp canines and retractable claws adapted for hunting and tearing flesh. Tigers play a vital role in maintaining the ecological balance by controlling herbivore populations in forests.

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.

An Atom bomb works on the principle of nuclear fission. In this process, the nucleus of a heavy atom such as Uranium-235 or Plutonium-239 splits into two or more smaller nuclei when bombarded with a neutron. This splitting releases a tremendous amount of energy along with more neutrons, which further cause chain reactions — leading to a massive explosion.

Other options (for context)

• Chemical reaction: Produces far less energy; used in conventional explosives, not atomic bombs.
• Nuclear fusion: Powers hydrogen bombs (thermonuclear bombs), not atomic bombs.
• Atomic collision: General term — does not describe the chain reaction mechanism in fission.

Explanation: The DNA sequencing technique was developed by Frederick Sanger in 1977. His method, known as the Sanger chain termination method, determines the nucleotide sequence of DNA using labeled dideoxynucleotides (ddNTPs) that terminate DNA chain elongation. This technique revolutionized molecular biology and became the foundation for genome sequencing projects such as the Human Genome Project.

Explanation: Measles is a highly contagious viral disease caused by the measles virus (a member of the Paramyxovirus group). It spreads through droplets released when an infected person coughs or sneezes. Symptoms include fever, cough, rash, and watery eyes. Other diseases listed — scurvy, asthma, and diabetes — are non-communicable as they do not spread from person to person.