What type of signals is essential for Sirius receivers to function effectively?

Sirius receivers are designed to operate using a combination of satellite and repeater signals, which is what makes option C the correct choice. Satellite signals are transmitted from orbiting satellites, delivering a wide array of channels that can be received across vast areas. These signals are crucial for providing coverage in areas where traditional radio signals may not reach, such as remote locations or urban environments with many obstructions.

Repeater signals, on the other hand, act to enhance the coverage and reliability of the satellite signals, particularly in areas where terrestrial obstructions might interfere with direct satellite communication. These repeaters relay the satellite signals, ensuring a consistent and clear audio experience for users.

In contrast, the other answer options focus singularly on types of signals such as analog, digital, or FM signals. While digital transmission is indeed used in Sirius' broadcasts, the system's effectiveness hinges on the integration of both satellite and repeater signals rather than relying exclusively on one type. Understanding this dual requirement is key for anyone operating or engaging with Sirius receivers, as it highlights the importance of a hybrid signal approach in modern communication systems.