IMO D....they beat in unison,yet adopt diff orientions..clearly defines the idea that cells beat together but they have diff orientations...So clearly showing contrast.
A) beating in unison and adopting - It shows some added info to chrac of cells , while it s not added, it is contrast.
(B) they beat in unison while adopting - no parllelism
(C) beat in unison, and adopt - same as A
(D) beating in unison yet adopting
(E) even though they beat in unison and adopt - Even though is wrong and parellism.

So, clearly D.

IMO: B
they correctly refer to cells,
beat in unison express the hamony while adopt express not unison or divert
while should be used here

Yes it is an old modifier SC but got confused between A and D becoz the contrast in D does not strike so easily. Thats why it is a toughie...

IMO: D.

What's the OA?