The correct answer is: The teacher repeatedly warned the students not to make noise, but the students did not obey.
The sentence is grammatically correct and makes sense. The teacher is warning the students not to make noise, but the students are not obeying. This could be because the students are not paying attention, or they are simply ignoring the teacher.
The sentence could be improved by adding more detail. For example, the teacher could be described as being frustrated or angry, and the students could be described as being disruptive or disrespectful. This would help to create a more vivid picture of the situation.
Overall, the sentence is well-written and conveys the intended message.