Which type of station is limited mainly due to the audio spectrum it reproduces?

The correct choice, AM station, is limited primarily by the audio spectrum it can reproduce due to the nature of amplitude modulation. AM broadcasting is designed to operate within a specific frequency range, typically from 530 to 1700 kHz. This modulation format inherently has limitations in the audio frequency response that can be effectively transmitted. AM stations are often subject to more interference and have a narrower bandwidth, typically able to transmit audio frequencies up to around 5 kHz. As a result, the sound quality of AM broadcasts is generally lower than that of FM stations, which can reproduce a wider audio spectrum of frequencies, up to 15-20 kHz, leading to clearer and higher-fidelity sound.

In contrast, FM stations utilize frequency modulation, which allows for a broader audio range and improved sound quality. Shortwave stations operate on higher frequencies, generally from 3 to 30 MHz, and are capable of transmitting over long distances, affecting neither audio quality nor the audio spectrum they reproduce. Broadcast station is a broader category that encompasses all forms of broadcasting, and as such, it doesn't specifically represent limitations related to audio spectrum reproduction. This clearly differentiates AM stations as the type that is confined by the audio spectrum they can deliver.