Answer:
a - continuity
Step-by-step explanation:
continuity is the unbroken and consistent process or existence or something over a period of time 
in this case, as you age your body begins to break down. since aging would be considered continuity the result (in this case) would be their reaction speed decreasing 
so since the overall effect or slower reaction time is a result of a continuing process this is an example of continuity 
hope this helps :)