The coastal sands of Kerala, particularly in the Kollam district, are rich in monazite sand. Monazite is a phosphate mineral that contains significant amounts of thorium, a naturally occurring radioactive element, leading to localized high levels of background radiation.

Itai-Itai disease, which translates to 'ouch-ouch' disease, is caused by chronic cadmium poisoning. It was first documented in Japan due to environmental contamination of water sources by mining activities. Cadmium accumulates in the kidneys and bones, leading to severe bone softening (osteomalacia) and kidney failure, characterized by intense pain in the joints and spine.

CFL lamps contain mercury vapor. Automobile batteries typically use lead plates. Polymers often contain phthalates as plasticizers. Diesel engines emit nitrogen oxides as a primary combustion byproduct. Thus, the correct mapping is a-4, b-3, c-2, d-1.

Thorium is significantly more abundant in the Earth's crust than uranium. When utilized in a breeder reactor, it provides higher energy density per unit mass. Furthermore, the thorium fuel cycle produces fewer long-lived transuranic waste products compared to the conventional uranium-plutonium cycle, making it a promising alternative for sustainable nuclear energy.

Humus is the dark, organic material that forms in soil when plant and animal matter decays. Chemically, it acts as an organic colloid, possessing a high surface area and negative charge, which allows it to hold onto essential nutrients and water, significantly improving soil fertility and structure.

An aerosol is a colloidal system consisting of fine solid particles or liquid droplets suspended in a gas, such as air. While aerosols can include liquid droplets, the provided option correctly identifies the presence of suspended solid particles as a primary characteristic of many atmospheric aerosols.

Acid rain is primarily caused by the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx). In the atmosphere, SO2 reacts with water to form sulfuric acid (H2SO4), which is the dominant contributor to the acidity of rain in most industrial regions, significantly lowering the pH of precipitation.

In the troposphere, the temperature typically decreases with increasing altitude. This rate of decrease, known as the environmental lapse rate, is approximately 6.4°C to 6.5°C per kilometer in the standard atmosphere.

CNG is cleaner because it burns more completely than petrol or diesel, resulting in significantly lower emissions of sulfur and nitrogen oxides. While it does emit carbon dioxide, it produces less per unit of energy compared to other fossil fuels, contributing to lower greenhouse gas emissions. Statement 2 is a consequence of 1 and 3.

Photochemical smog is often called 'brown air' due to the presence of nitrogen dioxide (NO2), which gives the smog a characteristic brownish haze. This type of smog forms when sunlight reacts with nitrogen oxides and volatile organic compounds in the atmosphere, common in urban areas with heavy traffic.