The correct answer is D: Man.

In the life cycle of tapeworms, humans (man) are typically definitive hosts, not secondary hosts. This means that humans are where the tapeworms reach maturity and reproduce. The secondary hosts are where the larvae develop into cysts before being ingested by the definitive host.

- A. Cow: Cattle can be a secondary host for Taenia saginata (beef tapeworm).
- B. Fish: Fish can be a secondary host for Diphyllobothrium species (fish tapeworms).
- C. Pig: Pigs can be a secondary host for Taenia solium (pork tapeworm).
- E. Dog: Dogs can be secondary hosts for Echinococcus granulosus (hydatid tapeworm).

Humans, however, are the definitive hosts in these tapeworm species, where the adult tapeworms live and reproduce. Hence, D: Man is the correct answer.