**Core Concept**
Diphyllobothrium latum, also known as the fish tapeworm, is a parasitic flatworm that causes diphyllobothriasis. This parasite has a complex life cycle involving multiple host species, including humans, fish, crustaceans, and birds.

**Why the Correct Answer is Right**
The second intermediate host of Diphyllobothrium latum is the copepod, a small crustacean that serves as an intermediate host in the life cycle of the parasite. The eggs of the parasite are ingested by the copepod, which then becomes infected. When a fish consumes the infected copepod, the parasite continues its life cycle.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the correct second intermediate host is the copepod, not the fish.
* **Option B:** This option is incorrect because the fish is actually the third intermediate host, not the second.
* **Option C:** This option is incorrect because the bird is a definitive host of Diphyllobothrium latum, not an intermediate host.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Diphyllobothrium latum is a zoonotic parasite, meaning it can be transmitted from animals to humans. Ingesting raw or undercooked fish contaminated with the parasite can lead to infection.

**Correct Answer: B. Fish.**