Given a = {a, e, i, o, u} and b = {a, l, g, e, b, r}, find a ∪ b.

{}
{a, e}
{a, b, e, g, i, l, o, r, u}

indicates you want the UNION of sets A and B.  So just make a list of elements that are in either set ("put all the elements into one 'box'").  Elements are listed only once; example:  a is in both sets, so put it in the union only once.

the 3rd choice, {a, b, e, g, i, l, o, r, u}