Knot is a unit of speed which of following?

The correct answer is Ship. A knot is a unit of speed primarily used in maritime and aeronautical contexts, historically and most famously associated with ships. One knot is defined as one nautical mile per hour (approximately 1.852 kilometers per hour or 1.151 miles per hour). The term originated from an old method of measuring a ship's speed by counting the number of knots on a line that ran out in a specific amount of time, as a log was trailed behind the vessel.

• Aeroplane is incorrect as the primary and historical association of the term "knot" is with marine navigation. While knots are indeed used in aviation for airspeed measurements, particularly in air traffic control and flight planning, the term's origin and most fundamental application are linked to ships.
• Train is incorrect because trains typically measure speed in kilometers per hour (km/h) or miles per hour (mph), not knots. There is no historical or practical precedent for using knots as a unit of speed for trains.
• Racing Car is incorrect because racing cars, like trains, measure speed in km/h or mph. Knots are entirely irrelevant to the speed measurement of land vehicles like racing cars.