Answer:
B.
Step-by-step explanation:
the geological time scale is basically just a 'Earth' dating system. geo-scientists use it to keep track of Earths geological/crusty/rocky events.
couldnt be 'A' because thats talking about geopolitical changes
cant be 'C' cause thats talking about a whole nother type of system called the geologic temperature record
'D' is talking about planetary equilibrium, that has to do with energy balance and radiation and the atmosphere and blah blah blah its not the right answer
the question was about geology and the only answer about basic geology is B. They define geological periods because scientists use them as reference points for tracking the history of the earth.