Given What We Know About How Classical Conditioning Occurs, Which

Given what we know about how classical conditioning occurs, which one of the following teaching strategies should be most productive?

A) Asking students to engage in activities that they can already do quite well, so that they never make mistakes.
B) Engaging students in classroom activities they find enjoyable.
C) Giving students numerous opportunities to explore the world around them in a relatively unrestricted fashion.
D) Making sure that desired student responses are always followed by reinforcing consequences.