Answer:
Option A, Valves
Step-by-step explanation:
Lymphatic vessels are valved structures as they carry thin walled valves in it. Valves allow one way movement of lymph fluid. As the pressure inside a lymphangion increases, the semilunar valve opens and allows the movement of lymph fluid into another lymphangion. This semilunar valve prevents the backward flow of the lymph fluid and thus a lymphatic system requires no central pump to operate efficiently.