In a research paper the main idea should be in the Introduction. So the correct answer is D. the introduction.
The introduction is the initial part of a text, whether in an essay, a book or a research article. In this first part the main idea is placed and usually expresses a summary of what will be explained or developed in the body of the text. In the introduction, the reader becomes familiar with the subject. After the introduction the body of the text is developed, also called "development" and, finally, the outcome, end or conclusion, depending on the type of text in question.
The structure or skeleton of a research paper is the following:
- the introduction (the main idea)
In an essay the main ideas of the introduction in the conclusion are retaken.