Final answer:
The breed of pony used for harness racing is the Hackney, known for their distinctive high-stepping gait. Other breeds like the Arabian, Clydesdale, and Shetland have different uses ranging from endurance riding to draft work.
Step-by-step explanation:
The breed of pony used for harness racing is the d) Hackney. Hackneys are known for their high-stepping gait, which is particularly sought after in harness racing and carriage driving. They are a breed that was developed in Britain for use as carriage horses and later became popular in harness racing events.
Arabian horses are known more for endurance riding and show ring events. Clydesdales are draft horses, often used for heavy pulling and farm work. Shetland ponies are small and used mainly for children's riding or as companion animals. Therefore, the Hackney is the correct answer since they are bred for performance in harness.