Final answer:
Vitamin B6 (option a) masks a vitamin B12 deficiency by alleviating some symptoms, but it does not fully address the underlying issue.
Step-by-step explanation:
The vitamin that masks a vitamin B12 deficiency is vitamin B6 (option a). Vitamin B6 can mask a vitamin B12 deficiency because both vitamins are involved in the production of red blood cells and proper functioning of the nervous system. However, it is important to note that while vitamin B6 can alleviate some symptoms of vitamin B12 deficiency, it does not fully address the underlying issue and should not be relied upon as a long-term solution.