Infectious diseases can be caused by air, water, food or any biological agent. Thus, nutritious food or staying away from infectious person can’t rule out the possibility of having the infection again. Exposure of B cell with antigen and its binding to antigen receptors stimulates B cell to produce plasma cells and memory cells. Upon the second encounter of the person with the same antigen, the already present memory cells divide quickly to produce more lymphocytes which in turn quickly produce antibodies. This kind of immune response selectively targeted against a particular antigen, to which body has previously been exposed, is known as acquired immunity and is responsible for preventing second-time infection. Thus, the correct answer is option A.