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

• A. in
• B. to
• C. at
• D. on

Answer: (D)

The main idea tested here is which preposition naturally links the noun “effects” to what is being affected. In English, when something has an impact on another thing, we use “effects on” to show that relationship. So in this sentence, the impact of lack of sleep is directed toward “our brain's ability to function,” making the phrase “effects on our brain's ability to function” the natural choice.

The other prepositions don’t fit the usual pattern: “in” would suggest inside something rather than affecting it, “to” isn’t used to express impact in this construction, and “at” isn’t used to link effects to their target in this context.