Hint: One could first think of the concept of Archimedes principle which has direct relation in the explanation of the concept of upthrust. Then you could explain the concept, and give the other name given to it. You could also give the real life examples related to this concept.
Complete step-by-step solution: In the question, we are to explain what is upthrust. Upthrust could be defined as the force that is acting upward on a body that is partially or completely submerged in a fluid. Another name given for this upward force is buoyancy. According to Archimedes principle, the weight of the object would be equal to the weight of the liquid displaced on this object being dropped in water. So, basically, for an object that is partially or completely submerged, there would be greater pressure at the bottom surface of this object compared to the pressure on the top of it. Hence, resultant force would thus be upwards. Also, if the upthrust is lesser compared to the object’s weight, then the object would sink. And when the upthrust is greater than the object’s weight it will float. Also, this fact has to do with density. Note: For the object with greater density than the fluid in which it is submerged, displacement of the fluid is not enough to keep the object floating and hence it sinks. On the other hand, when the density of the body is less than the fluid, it floats as the liquid.