Your supplier for a critical airplane component claim that the population mean strength of the supplies component is at lease 2.5 ksi. You conduct a hypothesis test at the 0.05 level of significance to try to disprove his claim(.i.e. your alternative hypothesis is that the mean is less than 2.5). The level of significance in this question represents:

a) the probability of a rejecting the supplier's claim when it is true

or

b) the probability of a rejecting the supplier's claim when it is false

I believe a is correct. This problem has been solved before with a as an answer but our TA during the review stated that b should be the answer. Can someone double check this one for me?