What is a quot;simple mode,quot; according to Locke?

What is a "simple mode," according to Locke?

(a) A simple idea formed by abstracting from the different possible degrees of a complex idea.

(b) A complex idea formed from the repetition and variation of a single simple idea.

(c) A simple idea of space, time, or number.

(d) The idea of a simple substance.

(e) A simplified idea formed by abstraction from the idea of a complex substance.