An effective introduction should
a. provide a detailed biography of the speaker.
b. comprise at least 25 percent of your total speech.
c. introduce the speech topic and purpose.
d. be prepared before the body of the speech.

C

Explanation: An introduccion should, as its name suggests, introduce to the topic we are about to talk about and what are the reasons behind whatever we are writting about.

A reader who reads the introduccion should know ,after having read it, what are we going to talk about and why we wrote about that topic. Some details might be included there but it have to be more as a summary.