Answer: One tail test.
Step-by-step explanation:
Given : A sample of 36 observations is selected from a normal population. The sample mean is 12, and the population standard deviation is 3.
Also, The set of hypothesis to conduct a test :-
The kind of test need to perform is dependent upon the alternative hypothesis.
Since, the alternative hypothesis is one tailed (right-tailed), so the test is one tail test.