Which of the following signals can a watchman use to signify a train is approaching?

The choice that includes both visual and audible signals is the correct answer because a watchman utilizes various methods to alert individuals of an approaching train effectively. This can include the use of a flag for visual signaling, as well as a whistle or horn for audible signaling.

The combination of both types of signals ensures that the warning can reach a wider audience, accommodating different situations such as varying visibility conditions or distances from the track. Visual signals can be important in daylight or clear conditions, while audible signals are helpful in situations where the visual element might not be seen immediately or some individuals might be out of sight of the watchman but still within hearing range.

Thus, the ability to use both visual and audible signals is essential for ensuring safety around railroad crossings and tracks, making this option the most comprehensive and effective in signaling the approach of a train.