Final answer:
A horse will most likely respond to a spur on the sensitive flank area by increasing speed or showing a startled reaction.
Step-by-step explanation:
The most likely response a horse will give to a rider spurring the sensitive area of the flank is one of increased speed or a startled reaction. Horses are sensitive creatures, and their flanks, being particularly sensitive areas, respond to light touches or pressures as cues from the rider. If spurred too harshly, the horse might react more dramatically, potentially leading to bucking or running to escape the discomfort.
The extracts provided illustrate the profound connection riders can have with their horses, and how a horse's muscular activity, responsiveness to rider commands, and unique sounds of hoofbeats are all parts of equine behavior.