**Answer:**

The correct answer is option **B) Archimedes principle**.

**Explanation:**

Archimedes principle states that when a body is submerged in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by the body. This principle was discovered by the ancient Greek mathematician and physicist, Archimedes.

**Key Points:**

1. **Buoyancy:** Buoyancy is the upward force exerted by a fluid (liquid or gas) on an object immersed in it. It is equal to the weight of the fluid displaced by the object.
2. **Weight of the Fluid Displaced:** When an object is submerged in a fluid, it displaces a certain volume of fluid. The weight of this displaced fluid is equal to the buoyant force acting on the object.
3. **Buoyant Force:** The buoyant force acts in the opposite direction to the force of gravity. It is responsible for making objects float or feel lighter when submerged in a fluid.
4. **Equilibrium Condition:** When an object is fully submerged in a fluid and is in equilibrium, the weight of the object is balanced by the buoyant force acting on it.
5. **Floating and Sinking:** If the weight of the object is less than the buoyant force, the object will float. If the weight of the object is greater than the buoyant force, the object will sink.

**Application of Archimedes Principle:**

Archimedes principle has various applications in engineering and everyday life, including:

1. **Ship Design:** The principle of buoyancy is crucial in designing ships and other floating vessels. By understanding the buoyant force acting on the ship, engineers can determine the appropriate shape and size of the hull to ensure stability and floatation.
2. **Submarines and Submersibles:** The principle of buoyancy is also used in the design and operation of submarines and submersibles. By controlling the buoyant force, these vessels can either float on the surface or submerge to different depths.
3. **Hot Air Balloons:** Hot air balloons work based on the principle of buoyancy. The hot air inside the balloon is less dense than the surrounding cold air, resulting in a buoyant force that lifts the balloon into the air.
4. **Swimming and Diving:** The buoyant force allows swimmers to float on the surface of the water. Divers can control their buoyancy by adjusting the amount of air in their buoyancy compensators, allowing them to ascend or descend in the water.
5. **Hydrostatic Testing:** Archimedes principle is used in hydrostatic testing to determine the volume and density of objects, such as pipes or tanks, by measuring the buoyant force acting on them when submerged in water.

In conclusion, the principle that states a submerged body is subjected to a buoyancy equal to the weight of the fluid displaced by it is known as Archimedes principle. This principle has numerous applications in engineering and everyday life, making it an essential concept in the study of fluid mechanics.