Okay, let's tackle this question about the major source of cyanocobalamin. First, I need to recall what cyanocobalamin is. Oh right, it's the synthetic form of vitamin B12. The question is asking where the primary dietary source is. I remember that vitamin B12 is found mainly in animal products. So, options like meat, fish, dairy, eggs would be the usual suspects.

Wait, the options aren't listed here, but the correct answer is probably an animal-based food. Let me think—common sources include liver, eggs, milk, and fortified foods. But the major source in a typical omnivore diet would be meat, especially organ meats like liver. However, in some regions, maybe dairy is more significant. For example, in some countries, people might consume more milk and cheese than organ meats.

But the key here is to identify the primary source. The body stores B12 in the liver, but dietary sources are crucial. Since B12 is produced by bacteria in the gut of animals, it's not found in plant foods naturally. So, the options are likely animal-based. If the correct answer is, say, liver, then the other options might be plant-based like legumes or grains, which don't have B12. Or maybe the options include fortified cereals, but those are synthetic additions.

The distractors could be options like green vegetables, which are a common trap for B12 sources, but actually, they don't contain it. Another distractor might be eggs, which do have some B12 but not as much as liver or meat. So the correct answer is probably an animal-based food, and the others are either plant-based or have lower amounts.

I need to make sure that the explanation covers the fact that B12 is exclusively from animal sources, the role of intrinsic factor in absorption, and why plant sources are incorrect. Also, mention that deficiency leads to pernicious anemia. The clinical pearl here is that vegetarians are at higher risk if not supplemented. The correct answer is likely an animal product, so the explanation should highlight that.

**Core Concept**
Cyanocobalamin (vitamin B12) is a water-soluble vitamin essential for DNA synthesis, red blood cell formation, and neurological function. It is synthesized exclusively by bacteria and found naturally only in animal-derived foods.

**Why the Correct Answer is Right**
The major dietary source of cyanocobalamin is **animal products**, particularly **liver**, **meat**, **eggs**, and **dairy**. The human body absorbs B12 via intrinsic factor in the ileum, and deficiency leads to pernicious anemia. Liver (especially beef liver) is the richest source, containing ~60-100 µg per 100g, far exceeding other foods.

**Why Each Wrong Option is Incorrect**
**Option A:** *Plant-based sources* (e.g., legumes, grains) are incorrect because B12 is not naturally present in plants.
**Option B:** *Fortified cereals* provide synthetic B12, but these are supplemental, not the "major" source in most diets.
**Option C:** *Dairy* contributes B12 but in lower amounts (1-3 µg per serving)

✓ Correct Answer: A. Animal only diet