Brown is not seen in a rainbow. A natural rainbow displays seven distinct colours — Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). These colours appear due to the dispersion of sunlight by water droplets in the atmosphere, but brown is not part of the visible spectrum formed by dispersion.

• Yellow, Green, Red: All are part of the seven colours of the rainbow.

Explanation: Antibodies (also called immunoglobulins) are specialized proteins produced by B-lymphocytes (a type of white blood cell) in response to the presence of foreign substances known as antigens (such as bacteria, viruses, or toxins). These antibodies recognize and bind specifically to the antigens to neutralize or destroy them, helping the body fight infections and build immunity. Antibodies are a vital part of the immune system and form the basis for vaccines and disease resistance.

All of the above are examples of static electricity. Static electricity is the result of the accumulation of electric charges on the surface of objects. When materials like glass, silk, ebonite, or wool are rubbed together, electrons are transferred, creating a charge imbalance. Similarly, lightning is a large-scale natural discharge of static electricity between clouds and the Earth.

• Glass rod & silk cloth: Positive and negative charges develop.
• Ebonite rod & wool: Negative charge accumulates on ebonite.
• Lightning: Discharge of atmospheric static electricity.

Explanation: The time period of a simple pendulum is given by the formula T = 2π√(L/g), where L is the length of the pendulum and g is the acceleration due to gravity. Since the Moon’s gravity is about 1/6th of Earth’s, the value of g decreases, causing the time period to increase.

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: Among the given particles, the proton is the heaviest. A proton is a positively charged subatomic particle found in the nucleus of an atom, with a mass of approximately 1.672 × 10⁻²⁷ kg. Although the neutron is slightly heavier than a proton, in most general questions like this one, protons are considered heavier in context when compared with electrons and neutrinos. Electrons are much lighter, with a mass about 1/1836 that of a proton, while neutrinos are nearly massless and electrically neutral.

A Hygrometer is an instrument used to measure the humidity (moisture content) in the atmosphere. It helps determine the percentage of water vapor present in the air, which is essential for weather forecasting, climate studies, and maintaining controlled environments in industries or laboratories. Some common types include the dry and wet bulb hygrometer and the digital hygrometer.

Other options (for context)

• Barometer: Used to measure atmospheric pressure.
• Anemometer: Used to measure wind speed.
• Thermometer: Used to measure temperature.

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.

Red light has the longest wavelength among visible colours, approximately 620–750 nanometers. Due to its longer wavelength, red light is scattered the least, which is why it is used in traffic signals and as warning lights for better visibility over long distances.

• Blue and Green: Shorter wavelengths, scattered more easily.
• Yellow: Intermediate wavelength between red and green.