**Core Concept**
Diphyllobothrium is a genus of fish tapeworms that can cause diphyllobothriasis, a parasitic infection in humans. The parasite is acquired through the consumption of raw or undercooked fish that contains the larvae.

**Why the Correct Answer is Right**
Diphyllobothrium species, particularly D. latum, are the largest tapeworms infecting humans. They can grow up to 10 meters in length and cause malabsorption of vitamin B12, leading to megaloblastic anemia. This occurs because the parasite competes with the host for vitamin B12, leading to a decrease in its absorption.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the characteristic of Diphyllobothrium. Without further information, it is difficult to evaluate the correctness of this option.
**Option B:** This option is incorrect because it does not accurately describe the mode of transmission of Diphyllobothrium. The parasite is not transmitted through contaminated water, but rather through the consumption of infected fish.
**Option C:** This option is incorrect because it does not accurately describe the clinical manifestation of Diphyllobothriasis. While malabsorption of vitamin B12 is a common manifestation, it is not the only one.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Diphyllobothriasis can be prevented by cooking fish thoroughly before consumption. Raw or undercooked fish can contain the larvae of Diphyllobothrium, which can lead to infection.

**Correct Answer:** D. Malabsorption of vitamin B12 is the primary cause of anemia in Diphyllobothriasis.