Which of the following is not classified as a natural cue in horse training?

In the context of horse training, natural cues are those that align closely with the horse's instincts and natural behaviors, typically relying on non-verbal communication. Natural cues include body language, which horses are very adept at interpreting, as they communicate through postures, movements, and expressions. Voice commands can also be considered a natural cue, as horses can learn to associate particular sounds or tones with specific actions or responses.

Cues from hands represent a physical form of communication that riders or trainers use, and while it can involve subtle movements that a horse may learn to respond to, it doesn't fall under the same category as instinctive cues.

On the other hand, a crop is an artificial tool used for directing or encouraging the horse; it does not represent a natural instinctive communication method. Instead, it introduces an element of pressure or stimulus that is not based on the horse's inherent understanding of cues. Thus, a crop is classified as a tool for training rather than a natural cue, making it the correct choice in this context.