Who/Whom Example Sentence

There are 2 clauses:

• Authoritative parents are more likely than permissive parents to have children.
  WHO as adolescents are self-confident, high in self-esteem, and responsibly independent.

Children is subject and i.e WHO is ok

doesnt it have to be a complete sentence? to have a subject?

It doesn't need to be that way, if so then you will never find Subject verb agreement issues in GMAT as there will be only one subject and one verb and there will no confusion in correcting those. GMAT sentences usually have many sub ordinate clauses but one main clause and each main and sub ordinate clause will be having a subject and object.

In the above case Authoritative parents are more likely than permissive parents to have children this is the main clause and WHO as adolescents are self-confident, high in self-esteem, and responsibly independent this is the sub ordinate clause.

You can check it out in any SC problem. Try finding the main and sub ordinate clauses and that will help you more to understand the concept. Even simple sentences have more than one clause.