We are given the following information:
Population mean = 18 inches
Population standard deviation = 2.5 inches
Sample size (n) = 50
Sample mean = 18.5
We are asked whether the data provide conclusive evidence that the training course is effective.
We should recall when a null hypothesis should be rejected--it is when the test value is less than the critical value. Here, our null hypothesis is that the training course is NOT effective.
In this case, since we are not provided the significance level, we will simply have to compare the population mean (critical value) and the sample mean (test value).
Because 18.5 (test value) > 18 (critical value), then we can reject the null hypothesis. In other words, we can conclude that the training is effective.
The answer is option D.