Archimedes’ major contribution/discovery was:
A. Photoelectric effect
B. Principle of buoyancy
C. Wave theory of light
D. Law of inertia

Hint: The principle with which we can measure the volume of liquid displaced when an object is dipped into it. It is the same principle with which we can calculate the upward thrust exerted by a liquid.

Complete step by step answer:
The theory of Archimedes makes it possible to measure the buoyancy of any moving object partially or completely immersed in a fluid. The downward force is literally its weight on the object. The force upward, or buoyant, on the object is that specified above by the theory of Archimedes. The net force on the object, then, is the difference between the measurements of the buoyant force and its weight.
The object rises if this net force is positive; the object sinks if negative; and the object is neutrally buoyant if zero; that is, it stays in position without either rising or falling. In plain terms, the theory of Archimedes states that when a body is partially or completely immersed in a fluid, it suffers an obvious weight loss proportional to the weight of the fluid replaced by the part(s) of the body(s) immersed. The theory of Archimedes is a law of physics basic to the dynamics of fluids. The theory of Archimedes discusses how ships float, submarines dive, fly hot air balloons, among many more examples.
To be specific, the photoelectric effect was discovered by Albert Einstein. Christiaan Huygens discovered the wave theory of light. Lastly, the law of inertia was discovered by Sir Isaac Newton.
From the above it is clear that Archimedes' major contribution/discovery was the principle of buoyancy.

So, the correct answer is “Option B”.

Note:
The theory of Archimedes is very beneficial for measuring an object's volume that does not have a normal form. It is easy to submerge an oddly shaped object, and the volume of the fluid displaced by the object is equal to the object's volume.