What is an quot;abstract idea,quot; according to Locke?

What is an "abstract idea," according to Locke?

(a) The idea of an immaterial object.

(b) An idea separated from all particular circumstances and made to serve as a representative of all its kind.

(c) A rational idea which can only be formed by philosophers, not by non-philosophers ("idiots").

(d) The idea of an object inaccessible to the senses, such as the idea of substance or the idea of God.

(e) None of the above.