If the cusps of an atrioventricular valve are damaged or functionally impaired so that they do not close completely during the normal point in the cardiac cycle, blood may flow backwards. this is called
b) regurgitation
c) thrombosis
d) automaticity
e) nodal depolarization

B) Regurgitation

Explanation:

Regurgitation refers to the backward flow of blood due to any reasons. Presence of atrioventricular valves between atrium and ventricle prevents the regurgitation, that is backward flow of blood. Structural or functional damage of any atrioventricular valves does not allow them to prevent the regurgitation.  

Example: Valvular insufficiency is the condition wherein the cusps of the valves (mostly the mitral valve) become stiff and leads to backward flow of blood.

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.

the answer is d. chordata