His studies of ice-polished rocks in his Alpine home land, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age
in which great ice sheets had existed in now currently temperate areas.(A) in which great ice sheets had existed in now currently temperate areas-> Do we need now currently.
(B) in which great ice sheets existed in what are now temperate areas-> look at first part "in which...in" and then present tense. It seems okay. Let's keep it.
(C) when great ice sheets existed where there were areas now temperate-> "were areas now temperate" It needs to be present tense.
(D) when great ice sheets had existed in current temperate areas-> had existing. Do we need past perfect here. Where is the second action.
(E) when great ice sheets existed in areas now that are temperate-> "areas now that are temperate" it doesn't make sense.
So, I think, it's B.