If s = {x, y,4, , which could describe u ?
a; u= { keys on a keyboard}
b; u= { letters }
c; u= { numbers }
d; u= { punctuation marks }

- A) U= { keys on a keyboard}

Explanation:-

Given set : S = {x,y,4,9,?}

Clearly it contains numbers (4,9), alphabets(x,y) and punctuation mark(?).

Thus the universal set of the given set must contain all of them.

Thus U={keys on a keyboard}