Which sentence exemplifies the proper use of the Oxford comma?

The correct choice demonstrates the proper use of the Oxford comma, which is the comma placed before the conjunction in a list of three or more items. In this case, the sentence "This screen has a pink, purple, and gray circle on it" includes commas after both "pink" and "purple," as well as before the conjunction "and." This structure clarifies that there are three distinct items: pink, purple, and gray. By using the Oxford comma, the sentence avoids any potential confusion about the number of items in the list and enhances readability.

The other choices do not correctly apply the Oxford comma. Some lack the comma before "and" or contain misplaced commas, which can lead to ambiguity and disrupt the clarity of the sentence. Overall, the choice effectively illustrates the importance of the Oxford comma in providing clarity in lists.