Choose the odd pair of words:

The first three pairs follow one pattern: a physical quantity and the unit used to measure it. Volume is measured in litre, length in metre, and resistance in ohm, the SI unit of electrical resistance.

Pressure : Barometer breaks that pattern. A barometer is an instrument, not a unit. It measures atmospheric pressure, and pressure itself is measured in pascal, bar or mmHg. So B is the odd pair.

• A: Litre is a unit of volume, accepted for use with the SI though not an SI base unit.
• C: Metre is the SI base unit of length.
• D: Ohm is the SI unit of resistance, named after Georg Simon Ohm.

Core concept: the difference between a unit and a measuring instrument, and the SI system. India follows the SI under the Legal Metrology Act, 2009, which connects this to standards and measurement topics in science and technology.

Trap: Aspirants often reject length : metre because metre is a base unit, or pick volume : litre because litre is not an SI base unit. First check the relationship type: quantity to unit, or quantity to instrument.