The correct answer is D. An unusual custom started.
Explanation
The passage talks about the custom of keeping money in pigs made of clay. In this, the author refers to the fact that this custom is associated with the pig representing greed. However, he expands information on how this unusual custom arose, alluding to the fact that the word "pig" sounds the same as the word "Pygg" used in England to refer to jars or pots made of clay in which people used to keep the money. Over time, the similarity of the words caused clay pigs to be made and given away as a joke. Eventually, this unusual custom spread around the world in places where different languages are spoken, and the origin of this tradition is unknown because of the English words "Pygg" and "pig". So the correct answer is D. An unusual custom started.