What is the subject complement in this sentence?

Painting is one of the oldest arts.

It can't be in the prepositional phrase.

That leaves

Painting is one

Which of those words is the subject complement?

I think it is one because painting is the subject?

Right.

The first thing you should do when analyzing a sentence is to note the prepositional phrases. Subjects, verbs, and complements are never in a prepositional phrase.

I taught my students to first put parentheses around all the prepositional phrases before finding the rest of the parts of the sentence.

http://www.chompchomp.com/terms/preposition.htm

What about this sentence

The subjects in early paintings were usually very important people.

The subject complement is people?

Right! :-)

Can u check 2 more for me.

Other subjects were historical or mythical figures.

Would it be figures.

Some paintings become huge, covering whole walls.

Would it be huge?

Both are correct.