Newton`s third law of motion states that for every action, there is an equal and opposite reaction. It means that when an object exerts a force on another object, the second object exerts an equal and opposite force on the first object.

Option 1: Force never occurs singly in nature. This statement is incorrect as there are cases where force can occur singularly, such as when no other objects are present. However, in the context of Newton`s third law, forces always occur in pairs.

Option 2: When the earth pulls a stone downwards due to gravity, the stone exerts a force on the earth. This statement is correct. According to Newton`s third law, when the earth pulls the stone downwards with a gravitational force, the stone exerts an equal and opposite force on the earth.

Option 3: There is a cause-effect relation implied in the third law. This statement is correct. The action and reaction forces in Newton`s third law occur simultaneously and are interdependent. One force is the cause, and the other force is the effect.

Option 4: There is no cause-effect relation implied in the third law. This statement is incorrect. Newton`s third law clearly establishes a cause-effect relationship between the action and reaction forces.

In conclusion