Final answer:
When a clock pendulum slows down and stops, it is most likely due to the force of friction acting on the pendulum.
Step-by-step explanation:
When a clock pendulum slows down and stops, the most likely reason is due to the force of friction acting on the pendulum. Friction opposes the motion of the pendulum, causing it to eventually come to a stop. This is in accordance with Newton's first law, which states that an object will remain at rest or continue at a constant velocity unless acted upon by an external force.