Which of newton's laws is related to the condition "A force cannot act alone"?

According to Newton's third law of motion, whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. These two forces are equal in magnitude but opposite in direction. For every action there is an equal and opposite reaction.

So a force cannot exist alone, and they always exist in pairs. This is related to Newton's third law of motion.