Which of Newton's laws states that:
An object at rest will stay at rest unless an external force acts upon it.
An object in motion won't change its velocity unless an external force acts upon it.

Newton's first law of motion states that an object at rest will stay at rest and an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
So the answer is Newton's First Law of Motion.
This law is also known as the law of inertia. Inertia is the tendency of an object to resist any change in its motion.
The first law of motion can be explained by the fact that every object has mass. Mass is the measure of an object's inertia. The more mass an object has, the more inertia it has.
When an object is at rest, it has no momentum. Momentum is the product of an object's mass and its velocity. The velocity of an object is its speed and direction of motion.
An object in motion has momentum. The more mass an object has, the more momentum it has. The faster an object is moving, the more momentum it has.
When an unbalanced force acts on an object, it changes the object's momentum. The force causes the object to accelerate. The acceleration of an object is the rate at which its velocity changes.
The first law of motion is a fundamental law of physics. It is used to explain the motion of objects in many different situations.