## **Core Concept**
The principle behind the floatation of a body in water is based on **Archimedes' Principle**, which states that an object will float if it displaces a volume of fluid (in this case, water) that weighs more than the object itself. This concept is crucial in understanding the buoyancy and density relationship.

## **Why the Correct Answer is Right**
The correct answer, **D. Buoyancy or upward force equal to weight of water displaced**, is right because according to Archimedes' Principle, the upward buoyant force on an object immersed in a fluid is equal to the weight of the fluid that the object displaces. If the weight of the fluid displaced is greater than the weight of the object, the object floats. This principle directly explains why bodies float in water.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Muscle relaxation* does not directly contribute to the floatation of a body in water. While muscle tone can affect the density of the body, floatation is primarily determined by the overall density of the body relative to water and the volume of water displaced.
- **Option B:** *Difference in density between body and water* is a related concept but not the direct reason for floatation. The actual mechanism is the buoyancy force resulting from the displacement of water.
- **Option C:** *Surface tension of water* plays a negligible role in the floatation of bodies. Surface tension can support small objects (like certain insects) on the water's surface but does not account for the floatation of larger bodies.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **density of the human body is slightly less than that of water**, which is why humans tend to float in water. This fact is essential in forensic science and understanding drowning.

## **Correct Answer:** D. Buoyancy or upward force equal to weight of water displaced.