It seems we must all sleep in ________ to maintain normal levels of skills.

• A. order
• B. manner
• C. way
• D. means

Answer: (A)

Expressing purpose with the fixed expression "in order to." In this sentence, the blank should be filled with "order" so the phrase reads "sleep in order to maintain..." that "in order to" is the standard way to say why something is done. The other options don’t fit because "in manner to," "in way to," and "in means to" aren’t natural English collocations after "sleep in" and don’t convey purpose correctly. So the best choice creates the clear, common purpose phrase: sleep in order to maintain normal levels of skills.