In the following question, select the one-word substitute for the description: “The upward force that a fluid exerts on a body floating in it.”

Introduction / Context:
This question combines basic physics with English vocabulary by asking for a one-word substitute that describes a well known physical phenomenon. In fluid mechanics, when a body is immersed in a liquid or gas, it experiences an upward force. Knowing the correct term “upthrust” is important not only for physics examinations but also for English questions that test technical vocabulary in science related contexts.

Given Data / Assumptions:
- Definition: “The upward force that a fluid exerts on a body floating in it.” - Options: Upthrust, Plunge, Submerge, Capsize. - We assume standard high-school level physics knowledge of forces in fluids. - The goal is to match the description with the precise technical term.

Concept / Approach:
According to basic physics, especially Archimedes’ principle, any body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced. This upward force is called “buoyant force” or “upthrust.” The other options describe different actions involving movement in water, such as plunging in, going underwater, or overturning, but none of them name the force itself. Therefore, the correct one-word substitute for this upward force is “Upthrust.”

Step-by-Step Solution:
1. Recall the physics concept: any object in a fluid experiences an upward push from the fluid. 2. Remember the technical vocabulary: that upward push is known as “buoyant force” or “upthrust”. 3. Analyse each option: - Upthrust: explicitly means the upward force exerted by a fluid on an immersed body. - Plunge: to jump or dive quickly into something, especially water. - Submerge: to put under water or go below the surface. - Capsize: to overturn a boat or vessel in the water. 4. Only “Upthrust” names the force described in the question; the others refer to actions or states of objects in water.

Verification / Alternative check:
Consider a simple experiment: if you push a ball down into water and release it, the ball rises to the surface. The force responsible for this rise is the fluid pushing upwards, which physics textbooks call “upthrust” or “buoyant force.” None of the other terms would be used as labels for the force in diagrams or formulae. This confirms that “Upthrust” is the correct scientific and linguistic match for the given description.

Why Other Options Are Wrong:
- Plunge: Describes the action of diving or falling quickly into water; it is a verb, not the name of a force. - Submerge: Means to go under the surface of water, again an action or state, not a force. - Capsize: Refers to the overturning of a boat or vessel, and is unrelated to the upward push exerted by a fluid.

Common Pitfalls:
Many students remember the concept but confuse the terms, mixing up words like “floatation”, “buoyancy”, and “upthrust”. While “buoyancy” refers to the phenomenon or ability to float, “upthrust” is specifically the upward force. In exams, reading the definition carefully and matching it to the most precise scientific term is crucial. Avoid picking verbs describing motion in water when the question clearly defines a “force”.

Final Answer:
The correct one-word substitute is Upthrust.