The right option is; C. "Your idea sounds fun, but I'd rather do something else."
Assertive communication is a type of communication skill in which an individual is able to express ideas, feelings and thoughts (both negative and positive) in an honest, direct and suitable way. Individuals with this type of skill stand up for their personal rights and express their views about something without being aggressive or upsetting others. From the question, “Your idea sounds fun, but I'd rather do something else” is an example of assertive communication. From the statement, the person’s view (response) about the other person’s idea is expressed in a calm and honest way.