As per the question the earth pulls the apple with a force of 0.5 N.
There are four options out of which we have to choose the most accurate answer.
From Newton's third law of motion we know that for every action there is an equal and opposite reaction.The force applied by one substance on other is called action and the force that it gets in turn is called reaction.
The earth pulls apple with a force of 0.5 N. Hence the apple must pull the earth with a force of 0.5 N. Otherwise it will violate the fundamental role of physics.
Hence the most accurate option is the last one.
