Final answer:
In a pendulum system, gravitational force affects the swinging motion, but neither gravitational nor electrostatic forces inherently decrease the pendulum's momentum. Electrostatic forces are generally not involved in a pendulum system unless it is a specialized setup.
Step-by-step explanation:
When considering a pendulum system, the force that is principal in its operation is gravitational force. Gravitational force is responsible for the pendulum's swinging motion. As the pendulum swings, its position changes relative to its equilibrium point. The components of gravitational force—perpendicular and parallel to the swing's motion—at different points in the swing, affect the pendulum's momentum differently. The parallel component of gravitational force acts to accelerate the pendulum towards its lowest point, increasing its speed and kinetic energy, and then decelerate it as it moves up the other side, but this process does not decrease its momentum overall; it just transfers it between potential and kinetic energy.
However, other forces such as air resistance (not mentioned in the options given) might decrease the pendulum's momentum over time. Electrostatic forces typically do not play a role in pendulum systems unless there is a specific setup that includes charged particles or fields. Hence, between the options given, neither gravitational nor electrostatic forces inherently decrease the momentum of a pendulum system (option D).