Answer: Newton's law of universal gravitation
Step-by-step explanation:
We know that an object does not change its state of rest or motion unless and until an external unbalanced force acts on it. Planets would have continued to move in straight path if they would have not pulled towards the Sun due to gravity. Gravitational pull of the Sun is the reason why the planets are pulled towards the Sun and orbit it.
Hence, Newton's law of universal gravitation which states that gravitational force exists between any two bodies having mass is the correct answer.