How does a dog most commonly acquire Dipylidium caninum?

Dipylidium caninum, commonly known as the cucumber tapeworm, is primarily transmitted to dogs through fleas. When a dog ingests a flea that carries the tapeworm larvae, typically while grooming itself, the tapeworm can develop into an adult in the dog's intestines. This lifecycle emphasizes the role of fleas as intermediate hosts in the transmission of the parasite to dogs, making flea control a crucial aspect of preventing dipylidiasis.

Unlike the other options, which either focus on transmission routes that are not relevant to this parasite or involve mechanisms that do not apply to Dipylidium caninum, the flea-mediated transmission is well documented and widely recognized in veterinary medicine. Therefore, understanding this specific route highlights the importance of maintaining flea control in preventing tapeworm infection in dogs.