The parachutelike membranes of Africa's scaly-tailed flying squirrels differ from those of the flying squirrels in the family Sciuridae because they are attached to a cartilage rod at the elbow instead of the wrist.
(A) because they are attached to a cartilage rod at the elbow instead of "at the elbow" is not parallel to "the wrist" - we need "at the wrist". Also, "because" shows a cause-effect relationship which is not being described here. The different attachment is not a reason for the membranes being different, only a description of how they are different. Eliminate.
(B) because of being attached to a cartilage rod at the elbow rather than at The same incorrect use of cause-effect expression as in (A). Also, even if we were to treat it as a cause-effect relationship (which it is not), the cause when using "because of" can only be a noun, not a clause. Eliminate.
(C) in their attachment to a cartilage rod at the elbow instead of being attached at "their attachment" is not parallel to "being attached". Also, wordy and unnecessarily complicated. Eliminate.
(D) in that they are attached to a cartilage rod at the elbow rather than at Correct answer - above errors are rectified and no new errors are introduced.
(E) in their being attached to a cartilage rod at the elbow instead of Same parallelism error as in (A). Eliminate.