:platelets in the blood pass by the ruptured sections of the blood vessel and adhere to exposed collagen fibers.

Adhesion

When a blood vessel ruptured, it will expose the collagen fiber into platelet. Platelet can adhere to the collagen fibers. This step is called adhesion.

After a while, it will be activated and slowly accumulate making a clot. This clot will slow the blood flow and might become a plug. 

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.