Final answer:
The only adjuvant used in human vaccines is alum (A), which is effective in triggering an immune response through the inflammasome mechanism.
Step-by-step explanation:
The only adjuvant that is used in vaccinating humans is alum (A). Alum salts are widely used as immune adjuvants because of their ability to trigger the inflammasome mechanism in cells, leading to the release of pro-inflammatory biomolecules and the activation of the immune system. Although substances like formaldehyde, mineral oil, saponin, and Freund's complete adjuvant have various applications in scientific processes and the development of vaccines, alum is the most common adjuvant actually used in human vaccines.