What is upthrust? Write its unit.

Hint:Upthrust is just a force. A force which operates when a body is placed into a fluid immersed fully or partially into that medium. It is associated with Archimedes’ principle. It is also known by the name buoyant force.

Complete step-by-step solution:
Buoyancy or upthrust is an upward force which is exerted by a fluid that opposes the weight of a partially or fully immersed object.
As a force its unit is $N$ known as $Newton$. Where $N = \dfrac{{kg.m}}{{{s^2}}}$.
 In a column of fluid if the pressure increases with depth as a result of the weight of the fluid. Thus, the pressure at the bottom of a column of fluid is greater than the top of the column. In this way, the pressure at the bottom of an object submerged in a fluid is also greater than at the top of the object. The pressure difference results in a net upward force on the object in the fluid.
The magnitude of the force is proportional to the pressure difference and explained by Archimedes’ principle, is equivalent to the weight of the fluid that would otherwise occupy the submerged volume of the object which is termed as the displaced fluid.
For this reason, an object whose average density is greater than that of the fluid in which it is submerged tends to sink. If the object is less dense than the liquid, the force can keep the object afloat

Note:Buoyancy also applies to fluid mixtures and it is the most common driving force of convection currents. In these cases, the mathematical way is changed but the principles remain the same. Examples of buoyancy driven flows include the spontaneous separation of air and water or oil and water. The center of buoyancy of an object is the center of gravity of the displaced volume of fluid.