The set that best describes the universal set U is; U = {keys on a keyboard }
We are told that S is a subset within a universal set U.
Now, from the subset S = (x, y, 4, 9, ?), we will need to find among the given sets the one that describes the best set.
From the subset, we see that the Universal set will be keys on a key board due to the fact that the keys on a keyboard will have alphabets, numbers and punctuation marks mentioned in the set.
Therefore U = {keys on a keyboard } will be the correct answer.
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