Answer:
Option). antigen.
Step-by-step explanation:
B cells are immune cells that form antibodies producing plasma cells. Antibodies are protein molecules that recognize foreign particles, such as bacterial or viral pathogens and provide immune responses.
The structures present on pathogens, which are recognized by specific antibodies are known as antigens, such as lipopolysaccharide molecules present in bacterial membrane.
Thus, the correct answer is option). antigen.