The (true) population mean is 12. You test the Null Hypothesis that the mean equals 12 at the 95% level of significance. The probability of making a Type 1 error is:

Explanation: A 95% level of significance means that if the Null is true, the probability of an extreme value of the estimator that causes the Null to be rejected is 100% - 95% = 5%. If this happens, we have a Type 1 error.