It is effective because it introduces the topic and previews the information that will follow.
Answer: Option C.
Step-by-step explanation:
The introduction of the essay should be such that includes in it the information about the topic and gives the readers a hint about the information that will be followed after the introduction section.
In the introduction about the United Nations given by Farhad, he has told the reason why there was a need of such an organisation in the world. So in the section to be followed, he will tell about the further information of this organisation. Hence it is effective.