Final answer:
A pulsating brake pedal in a disc brake system is typically caused by a warped brake rotor which results from uneven contact between the brake pads and the rotor surface.
Step-by-step explanation:
The likely cause of a pulsating brake pedal in a disc brake system is a) Warped brake rotor. When a brake rotor is warped, it causes the brake pads to contact the rotor unevenly as it turns, which leads to the pulsation feeling in the brake pedal. This pulsation can be felt when the brakes are applied, and it is a common sign that the rotor has become uneven due to overheating or other types of wear.