This question contains error of parallelism, which is very subtle , and so might hide the answer..

Here the phrases ''ranging from'' and ''to the staffing of'' should be parallel...
the point to note is that 'ranging from' applies to (duty assignments in nuclear submarines and air-traffic control towers ) only...
and ''staffing from'' explains a different function altogether,,

hope it helps...

gmat740 wrote:Studies of the human “sleep-wake cycle” have practical relevance for matters ranging from duty assignments in nuclear submarines and air-traffic control towers to the staff of shifts in 24-hour factories.
(A) to the staff of
(B) to those who staff
(C) to the staffing of
(D) and staffing
(E) and the staff of


[spoiler]OA-C
IMO-a[/spoiler]

IMO C should be better than A
from X to Y
X and Y here represents same type of actions, 'duty assignments' and 'staffing of shifts' respectively
In A, staff is a noun which doesn't match with the action after 'from' in the non-underlined portion