In expository writing - writing that explains - the main idea of a passage is usually stated in the passage; it is explicit. Frequently, it will be stated as the first sentence of the passage or at the end of the introduction. In your own writing, you may have learned to call this main idea a topic statement. Sometimes, however, particularly in narrative writing - writing that tells a story - the main idea is implicit, unstated. When you cannot find an expressly stated main idea, you must figure it out or infer it, taking care that you have considered the entire passage.
You can help yourself by asking,
If you state the main idea in a single sentence before you read the choices given by the test makers, it should clarify your own ideas and, therefore, be able to answer the question more easily.