Name of the force exerted by a fluid on objects immersed in it

Answer: Buoyant force

It is the force the fluids exert in a submerged body or object, and is directed upwards. This is due to the difference in pressures between the lower part and the upper part of the submerged object.

This is also in accordance with the Archimedes' Principle:

A body totally or partially immersed in a fluid at rest, experiences a vertical upward thrust equal to the mass weight of the body volume that is displaced.

It should be also noted that the density of a body is related to its buoyancy, a substance or body will float on another fluid if its density is lower.