ASSERTION: The familiar equation $mg = R$ for a body on a table is true only if the body is in equilibrium.
REASON: The equality of $mg$ and $R$ has no connection with the third law.
(A) both assertion and reason are correct and reason is the correct explanation for assertion
(B) both assertion and reason are correct but reason is not the correct explanation for assertion
(C) assertion is correct but reason is incorrect
(D) both assertion and reason are incorrect

Hint: The solution can be determined by using the statement of the newton’s third law of motion. Because, in Newton's third law of motion, it gives the statement about the force and reaction force, then the solution can be determined.

Complete answer:
 Newton's third law of motion states that, for every action there is an equal and opposite reaction. Then for every force there is an equal and opposite reaction force. The force of the object on the table is equal to the product of the mass of the object and the acceleration due to gravity on the object. The reaction force $R$ is given by the table on the object is equal to the amount of force given by the object on the table. Then the object on the table is said to be in equilibrium.
This stamen is given by the newton’s third law of motion, therefore the third law of motion has the connection with the equality of the $mg$ and $R$, so the assertion is correct and the reason is incorrect.

So, the correct answer is “Option C”.

Note:
When the two objects are colliding with each other the force exerted by one object is equal to the force exerted by the other object, but the force exerted by the two objects are in the opposite direction. In this case, the force acts on the horizontal plane. When the object is placed on the table, the force on the table due to the object is equal to the force on the object by the table, but in the opposite direction, in this case, the force acts in a vertical direction.