Final answer:
When no outside forces are acting, the graph of an object's momentum as a function of time would be a horizontal line.
Step-by-step explanation:
A graph of the momentum of an object as a function of time, when no outside forces are acting, would be a straight line. This is because according to Newton's first law, an object in motion will stay in motion with a constant velocity unless acted upon by an external force. In this case, the absence of external forces means that the momentum of the object will remain constant over time. Therefore, the graph of momentum vs. time would be a horizontal line.