The answer is D. Pistol : Gun.
A church is a building for religious activity, a monument is a structure built to commemorate a person or event, a car is a vehicle for transportation, and a bus is a large vehicle for public transportation. A pistol is a small firearm, while a gun is a larger firearm.
Pistol and gun are both firearms, but they are different in size and power. A pistol is a small firearm that is typically held and fired with one hand, while a gun is a larger firearm that is typically held and fired with two hands. Pistols are typically used for self-defense or target shooting, while guns are typically used for hunting or warfare.
In the given options, all the other words are related to each other in a way that pistol and gun are not. Church and monument are both structures that are built to commemorate something, car and bus are both vehicles for transportation, and pond and lake are both bodies of water.