The complement of a set would contain all elements (in the universal set) that are NOT in the set itself.

Let U​ = ​{1​, 3​, 4​, 7​, 8​, 11​, 15​, 16​, 18​}

S = {3, 4, 8, 15, 18}

Answer:

S' = {1, 7, 11, 16}

Complement of a set is all the elements in set U that are not in given set { 3, 4, 8, 15,18 }

so complement set is {1,7,11,16}