I think the correct answer would be the third option. Using an implied main idea would not be a good strategy for writing a supporting sentence. These sentences should contain substance or evidences that would support the main idea of what you are writing. Using implied ideas would be very vague for the readers making the readers doubt on what message you are conveying. Supporting sentences should be straight to the point and be very specific and should provide a good foundation for the main idea. Using facts, examples and statistics that are verified by reliable people are very good strategies in writing supporting sentences.
