As the name itself says, subject complements are words which complete a subject. They are always preceded by a linking verb in a sentence (a linking verb connects two parts of a sentence). Here is an example of a subject complement:
He is a <u />good <u>boy.
</u>The subject is HE; the linking verb is IS; and the subject complement is BOY.<u>
</u>
If you're asking for main idea, the main idea or thesis statement sentence is sentence 1, where they explain what the passage is about.