had existed is not correct. it should be simple past.
so A and D are out.
Where there were in C is too wordy
in E now should modify the temperate and should be put close to it.
B is correct
Bests
Arash

His studies of ice-polished rocks in his Alpine homeland, 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.

Simple form-
X led Louis in 1837 to propose the concept of an age in which great ice sheets had existed in now currently temperate areas.

Two signs - "usage of had" and weird fragment "now currently temperate areas". ('Now' and 'currently' mean the same things)

When I look at the options, I found another split - usage of "in which" and "when". "In which" sounds better because "when" here indicates that two things happen parallel.

For example - the concept of age when sunny took GMAT.

(A) in which great ice sheets had existed in now currently temperate areas
Redundant (Now and currently). OUT
(B) in which great ice sheets existed in what are now temperate areas
This is the answer.
(C) when great ice sheets existed where there were areas now temperate
Wrong usage of when. OUT
(D) when great ice sheets had existed in current temperate areas
Wrong usage of when. OUT
(E) when great ice sheets existed in areas now that are temperate
Wrong usage of when. OUT