Going forward, please ensure you post the complete question.

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 what are now temperate areas

(A) in which great ice sheets had existed in what are now temperate areas
(B) in which great ice sheets existed in what are now temperate areas
(C) when great ice sheets existed where there were areas now temperate
(D) when great ice sheets had existed in current temperate areas
(E) when great ice sheets existed in areas now that are temperate

hanspaul wrote:Can someone explain a few things about "What"

1) Is what a modifier? In some cases it looks like it can replace "which" or "that"
2) Does to modify a noun or a clause?

From OG:

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 existed in what are
now temperate areas

IMO B




OA B

Thanks for the reminder pesfunk.

hanspaul wrote:Can someone explain a few things about "What"

1) Is what a modifier? In some cases it looks like it can replace "which" or "that"
2) Does to modify a noun or a clause?

From OG:

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 existed in what are
now temperate areas

"What" is often a pronoun, and as such will work similarly to "that" or "which" (and in fact often means "those which" or "that which", as in "He knows what he wants").

"What" can also be used as an adjective, meaning (among other things) "the [noun] that", as in "They ate what berries they found" (i.e. "they ate the berries that they found")