Lack of sleep has serious effects ________ our brain's ability to function.

• A. on
• B. in
• C. to
• D. at

Answer: (A)

The main idea is choosing the preposition that shows what is being affected. After “effects,” we typically use “on” to indicate the thing that is impacted. In this sentence, the thing affected is our brain’s ability to function, so “effects on our brain’s ability to function” is natural and correct. Other prepositions don’t fit because they don’t pair with “effects” in this sense (for example, “in” would not correctly show the target of the impact, and “to” or “at” aren’t used with this phrase).