Final answer:
The type of fencing deterrent that provides a nonaggressive barrier to deter birds, such as pigeons, at the top of a fence is known as Pigeon spikes.
Step-by-step explanation:
Types of fencing deterrents include the use of Pigeon spikes, which provide a nonaggressive barrier at the top of a fence. When considering options such as Barbed wire, Razor wire, and Spike strips, these are aggressive barriers designed to prevent human intrusion. However, Pigeon spikes are designed to deter birds without causing serious harm, making them the correct answer (D) to the question as they offer a more humane and nonaggressive method of deterrence, specifically for pigeons and other birds.