Studies of circadian rhythms can be applied to problems ranging from chronic insomnia in frequent transcontinental air travelers to workers in all-night plants being inefficient.

A question of parallelism and comparison

(A) to workers in all-night plants being inefficient -- 'workers' is the wrong comparison; use of being as a modifier is wrong.
(B) toward workers in all-night plants being inefficient -- 'workers' is comparison error.

(C) To workers in all-night plants who are inefficient-- 'workers' is problematic and 'plants who' is wrong.

(D) to combat worker inefficiency in all-night plants - to combat is an infinitive that won't go with insomnia.

(E) to decreased efficiency among workers in all-night plants -- the best choice
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Here from X to Y idiom.
In nonunderlined portion from is accompanied with noun so we need noun with "TO".
A, C, and E filters out.

Now A is out because of wrong comparison, use of being, and wrong use plant being inefficient .
C is out for use of relative pronoun who for plant. Complete disaster.

E clearly wins.

Studies of circadian rhythms can be applied to problems ranging from chronic insomnia in frequent transcontinental air travelers to workers in all-night plants being inefficient.

(A) to workers in all-night plants being inefficient
(B) toward workers in all-night plants being inefficient
(C) to workers in all-night plants who are inefficient
(D) to combat worker inefficiency in all-night plants
(E) to decreased efficiency among workers in all-night plants

Answer is E --- to-to parallelism + plural-plural
to problems----to workers ----------incorrect
to problems---to deceased ---- correct