The body system on the chart
Newton's second law states that the resultant of the forces applied to an object is equal to the product between the object's mass and its acceleration:

where in our problem, m is the mass the (child+cart) and a is the acceleration of the system.
We are only concerned about what it happens on the horizontal axis, so there are two forces acting on the cart+child system: the force F of the man pushing it, and the frictional force

acting in the opposite direction. So Newton's second law can be rewritten as

or

since the frictional force is 15 N and we want to achieve an acceleration of

, we can substitute these values to find what is the force the man needs:
You have no options here so I'll just answer. It can cause a rise in heart rate and greatly increases the risk of overheating and even death. If you grab the rabbit too hard, you risk breaking/fracturing a bone or causing other kinds of damage, whether externally or internally, to the rabbit.