The universal set U from which the subset was derived is; U = {keys on a keyboard }
We are told that S is a subset within a universal set, U.
Now, If S = {x, y, 4, 9, ?}
Given S is a subset within a universal set U;
According to the question S = {x, y, 4, 9, ?}.
Now, we need to find among the given sets the one that describes the best set.
The best Universal set will be {keys on a key board } since the keys on a keyboard will have alphabets, numbers and punctuation marks mentioned in the set
Therefore, U = {keys on a keyboard }
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