Which of the following types of signal has the narrowest bandwidth?

The type of signal that has the narrowest bandwidth is Continuous Wave (CW). This is because CW transmission consists of sending a continuous signal at a single frequency, which represents a binary state of on and off (sending a tone or no tone). Since CW does not carry audio or complex information, its bandwidth is determined primarily by the keying speed.

In practice, the essential bandwidth for CW operation is quite narrow, often estimated to be around just a few Hertz. This narrow bandwidth allows CW signals to be transmitted and received over great distances with minimal interference from other signals, making CW a very efficient mode for amateur radio operators.

In contrast, FM voice and Single Sideband (SSB) voice transmissions require much wider bandwidths to accommodate the frequency modulations and the range of audio frequencies they carry. Slow-scan TV also requires a broader bandwidth compared to CW, as it transmits moving images which necessitates a more considerable data rate to effectively convey the visual information. Thus, CW's unique operation as a purely on-off keyed signal makes it the mode with the narrowest bandwidth.