I believe that the answer is:
C.(Hydration)
Hope this helps!
The repetition of the word knock, first of all, conveys a specific literary device which is called Alliteration. Here is the definition of alliteration: It is the appearing of the same, exact letter or sound at the starting point of adjacent or closely linked words. Secondly, the repetitious sound of knocking , conveys the knocking as continuous. Hope that this answers your question.
B. the specifically ivention is called :Water frame
I'd say it was D or the last choice. It makes most sense to me.