Which of the following is NOT a natural riding aid?

The correct choice highlights that spurs are not considered a natural riding aid. Natural riding aids are those that a rider uses instinctively and without any additional equipment; they involve the rider's body and voice as the primary means of communication with the horse.

The rider's voice serves as a natural aid by providing vocal cues that can calm, encourage, or prompt the horse to respond. The rider's legs are also a natural aid, used to apply pressure or support, signaling the horse to move forward or adjust its pace. Additionally, the distribution of the rider's weight is crucial in guiding the horse, as shifting one's weight can indicate direction or balance shifts.

In contrast, spurs are an artificial aid designed to enhance the rider's ability to communicate with the horse through the application of pressure to the sides. They are used to reinforce cues that may otherwise be insufficient, making them a modified tool rather than a natural aspect of riding. Therefore, spurs stand apart from the other aids listed, which are innate to the rider's body.