What is the main cause of lung cancer?

The correct answer is Option (4) → Chemical carcinogens present in tobacco smoke.

Agents that turn normal cells into cancer cells are called carcinogens. They may be physical (radiations), chemical or biological (viruses).

Option (1) Exposure to ionising radiation. — Incorrect. Ionising radiations such as X-rays and gamma rays damage DNA and can cause cancer, but they are not named as the main cause of lung cancer.

Option (2) Genetic factors. — Incorrect. Inherited genes can make some people more likely to get certain cancers, but they are not the main cause of lung cancer.

Option (3) Consumption of alcohol. — Incorrect. Heavy use of alcohol mainly damages the liver (cirrhosis) and the nervous system, not the lungs.

Option (4) Chemical carcinogens present in tobacco smoke. — Correct. Tobacco smoke carries chemical carcinogens straight into the lungs, where they change the cells lining the lungs into cancer cells.

NCERT Extract:
Cancer – “… Causes of cancer : Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical or biological agents. These agents are called carcinogens. Ionising radiations like X-rays and gamma rays and non-ionizing radiations like UV cause DNA damage leading to neoplastic transformation. The chemical carcinogens present in tobacco smoke have been identified as a major cause of lung cancer.”