Final answer:
The correct answer is 'a. Newton's 1st Law' because it relates to inertia, which is the property that makes it harder to push an adult on a swing than a child due to the adult's greater mass.
Step-by-step explanation:
Pushing an adult on the same swing is indeed harder because the adult has more inertia, which refers to the tendency of an object to remain at rest or to continue moving at a consistent speed in a straight line unless acted upon by an external force. This principle relates to Newton's First Law of Motion, often called the law of inertia. So, the correct answer to the question is a. Newton's 1st Law.
An adult has more mass than a child, and since inertia is directly related to the mass of an object, an adult on a swing will have more inertia and therefore will resist changes to their motion more than a child would. This is why it's easier to push a child on a swing than it is to push an adult.