Which one of the following sets of elements was primarily responsible for the origin of life on the Earth?

The correct answer is B, Carbon, Hydrogen, Nitrogen. These are the core elements of the organic molecules, such as amino acids, nucleotides, sugars and lipids, from which the first living systems were assembled. The Miller-Urey experiment of 1952 showed that simple gases containing C, H and N, under an energy source, can yield amino acids, which is why CHN is central to the origin-of-life discussion.

• A is wrong. Hydrogen and oxygen are important in water, but sodium is an ion used in later cellular function, not a building block of early organic molecules.
• C is wrong. Calcium and phosphorus matter in biology (phosphorus is in nucleic acids and ATP), but the set lacks carbon, the backbone of every organic molecule. Free oxygen was also scarce in the early reducing atmosphere.
• D is wrong. Potassium is an essential ion for nerve signalling and cell function, not a structural element of early life. The set also omits nitrogen, which is needed for amino acids and nucleic acid bases.

For Mains, this links to the chemical evolution theories (Oparin-Haldane) and to the broader question of how life might arise elsewhere, which is often asked in the context of astrobiology.

Trap: Choosing the set with phosphorus or oxygen because they are biologically vital today. The question asks about origin, so look for the elements that form the organic framework.