In astronomy the term “red shift” denotes the extent to which light from a distant galaxy has been shifted toward the red, or long-wave, end of the light spectrum by the rapid motion of the galaxy away from the Earth.
(A) to which light from a distant galaxy has been shifted
(B) to which light from a distant galaxy has shifted
(C) that light from a distant galaxy has been shifted
(D) of light from a distant galaxy shifting
(E) of the shift of light from a distant galaxy


My answer: A

light has been shifted ... by

IMO "A" is best...since idiom is "extent to which....

we need been since there is "by" in non underlined portion.