**Core Concept**
Vitamin B12 is an essential nutrient that plays a critical role in the synthesis of DNA, fatty acid, and amino acid metabolism. It is a complex molecule that requires specific dietary sources to maintain adequate levels in the body.

**Why the Correct Answer is Right**
Vitamin B12 is primarily found in animal-derived foods due to its synthesis by microorganisms in the gastrointestinal tract of animals. Soya bean, being a plant-based food, does not contain vitamin B12, making it an essential dietary source to be avoided in this context. The correct answer is soya bean because it lacks the necessary bacterial flora to produce vitamin B12.

**Why Each Wrong Option is Incorrect**
**Option A:** Fish, while not a rich source, does contain some vitamin B12, particularly fatty fish like salmon and sardines. However, it is not the best source among the options provided.
**Option B:** Meat, including beef, pork, and lamb, is an excellent source of vitamin B12 due to the presence of bacteria like *Helicobacter pylori* in the gastrointestinal tract of animals.
**Option D:** Liver, particularly organ meats like beef liver, chicken liver, and fish liver, is an excellent source of vitamin B12.

**Clinical Pearl / High-Yield Fact**
Vitamin B12 deficiency can lead to megaloblastic anemia, neurological symptoms like numbness and weakness, and cognitive impairment. It is essential to maintain adequate dietary sources or consider supplements, especially in individuals with vegetarian or vegan diets.

**✓ Correct Answer: C. Soya bean**