The argument concludes that humans must eat red meat to prevent vitamin B12 deficiency. The assumption underlying this argument is that there are no other practical alternative sources of vitamin B12 for humans.

Looking at the answer choices:
- The argument would be undermined if humans could prevent deficiency by using supplements or other sources.
- Choice A ("Humans who do not consume red meat could not prevent vitamin B12 deficiency by using vitamin B12 supplements.") is an assumption required for the argument: If supplements could prevent deficiency, eating red meat would not be the only way. mayoclinic +2

The other options introduce unrelated issues (other foods, absorption, effects of deficiency, avoidance of red meat) that do not directly support the logic of the argument.

Correct answer:
A) Humans who do not consume red meat could not prevent vitamin B12 deficiency by using vitamin B12 supplements.