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.
Age in which AND age when both are fine in this sentence.
(A) in which great ice sheets had existed in now currently temperate areas
Had existed is not correct in this sentence since the two events that are sequenced are not related.
(B) in which great ice sheets existed in what are now temperate areas
This is the correct choice.
(C) when great ice sheets existed where there were areas now temperate
Where there were areas – changes the meaning of the sentence.
led Louis Agassiz in 1837 to propose the concept of an age when great ice sheets existed - may mean that ice sheets existed in 1837 when the concept was proposed.
(D) when great ice sheets had existed in current temperate areas
The use of had is incorrect in this sentence.
(E) when great ice sheets existed in areas now that are temperate
Are now that are temperate should be area that are now temperate.
led Louis Agassiz in 1837 to propose the concept of an age when great ice sheets existed - may mean that ice sheets existed in 1837 when the concept was proposed.