In immunology, the term adjuvant refers to a substance that:

Correct option is B

An adjuvant is added to vaccines to enhance the immune response by increasing antigen recognition and prolonging antigen presentation. Example: Alum.
Explanation of options: (a) ✘ Adjuvants do not reduce side effects; they can actually increase local reactions. (b) ✔ Correct; they enhance the immunogenicity and effectiveness of the vaccine. (c) ✘ They do not convert vaccine type; live/inactivated is fixed. (d) ✘ Preservatives maintain stability, not immune effectiveness.