Which Statement Best Describes an Account of Immunological Memory

Which statement best describes an account of immunological memory?

A) the human body's ability to distinguish self from nonself
B) the observation that some strains of the pathogen that causes dengue fever cause more severe disease than others
C) the ability of a helper T cell to signal B cells via cytokines
D) the observation that someone who had recovered from the plague could safely care for those newly diseased