Final answer:
The correct answer is option D. To ensure adequate vitamin B12 absorption, a person needs an acidic stomach environment, intrinsic factor, and sufficient dietary intake of B12. These factors work in conjunction to allow proper uptake of B12 in the small intestine.
Step-by-step explanation:
In order to ensure you have adequate vitamin B12, you need A. stomach maintains an acidic environment, B. intrinsic factor, and C. adequate B12 intake.
Hence, the correct answer is D. All of the above. The stomach's acidity helps to release vitamin B12 from the food you eat, making it available for absorption.
The intrinsic factor is a protein secreted by the stomach that binds to vitamin B12, forming a complex that protects the vitamin as it moves through the digestive system.
Finally, the complex is absorbed in the small intestine, specifically in the terminal ileum. Without adequate intake, even with stomach acidity and the intrinsic factor, the body cannot maintain sufficient B12 levels.