Final answer:
Decreasing the confidence level from 90% to 85% makes the confidence interval narrower, not wider, so the correct answer is 'd) none of the above.'
Step-by-step explanation:
When you decrease the level of confidence from 90% to 85%, the width of the confidence interval actually decreases. This is because lower confidence levels correspond to a smaller error bound. At a lower confidence level, you're essentially saying that you are requiring less certainty that the true parameter (mean, proportion, etc.) is within the interval, which means the interval can be narrower while still capturing the true parameter with the specified level of confidence.
In other words, as the confidence level decreases, the confidence interval becomes narrower, not wider, because you are encompassing a smaller area under the normal distribution's curve. The correct answer to the student's question would, therefore, be 'd) none of the above.'