Question

Where is the main idea of an essay introduced to the reader? in the topic sentence in the source citation in the thesis statement in the research question

Answer 1

Answer: the answer would be the thesis statement, as it is a statement which summarizes the main idea of the essay as a whole.

Explanation: the topic sentence is the sentence which describes the main idea of the paragraph, not the essay.  the source citation is where you cite your sources if you have any, and the research question is the question you are asking yourself to help with your researching.