To solve this question, the user needs to understand prepositions and their usage. Prepositions are words that show the relationship between a noun or pronoun and other words in a sentence.

Each of the options can be used as a preposition, but the correct answer in this sentence is the option that best describes the location of the keys.

A. "At" is a preposition that indicates a specific location, but it is not the best option in this sentence. It would be more appropriate to use "at" if the sentence was "I left the keys at the office."

B. "In" is a preposition that usually describes the location of an object inside something else. For example, "The keys are in the drawer." However, it is not the best option in this sentence because the keys are not inside anything.

C. "On" is a preposition that describes a location on a surface. This option is the best choice because the keys are most likely resting on the surface of the kitchen counter.

D. "None of these" is not the correct answer as option C, "on," is the most appropriate preposition in this sentence.

The Answer is: C