The correct options are
A {a, e}
B { }
D {a, e, i, o, u}
Set A = {x: x is a vowel} = {a, e, i, o, u}
Subsets of set A are :
{ }, {a}, {e}, {i}, {o}, {u}, {a, e}, {a, i}, {a, o}, {a, u}, {e, i},{e, o}, {e, u}, {i, o}, {i, u}, {o, u}, {a, e, i,}, {a, e, o}, {a, e, u}, {a, i, o}, {a, i, u}, {a, o, u}, {e, i, o}, {e, i, u}, {e, o, u}, {i, o, u}, {a, e, i, o}, {a, e, i, u}, {a, e, o, u}, {a, i, o, u}, {e, i, o, u}, {a, e, i, o, u}.
⇒ In the given options {}, {e, i, a, o, u} and {a, e} are subsets of set A.