<p>@chocolatecricket - The answer is D. You use the t-distribution if the population standard deviation is unknown.</p>
<p>@zzxjoanw3 The margin of error is 3, so 19-3 would be 16, and 19+3 is 22. You are 95% confident that the true mean is between 16 and 22, so it is unlikely that any value out of that range will be the true mean. 15 is out of that range, so the answer is E.</p>