"Comma which modifier"

[ShivamD481]

I recently completed a GMAT Prep Exam and come across this question:

"In attempting to solve the problems caused by a lowering of the price of oil, oil companies operating in the North Sea have taken a variety of approaches, which includes their reducing employment, using new technology to pump oil more efficiently from smaller fields, and finding innovative ways to cut the cost of building and operating platforms."

(A) which includes their reducing employment, using new technology to pump oil more efficiently
(B) which includes reducing employment, using new technology to be more efficient in pumping oil
(C) which include reducing employment, using new technology to pump oil more efficiently
(D) which include the reduction of employment, their using new technology to pump oil more efficiently
(E) including a reduction of employment, their use of new technology to be more efficient at pumping oil

I immediately went to answer (E) because I thought the "comma which" was incorrect. Can someone explain when "comma which" is allowed to be used, and when it is strictly forbidden. Thank You

[Sage Pearce-Higgins]

Please be sure to search the forums before posting: https://www.manhattanprep.com/gmat/foru ... tml#p30111