Question 867794
Some books vary (since it's not a hard fast rule), but in most cases I've seen that you use the t distribution if you don't know the population standard deviation and n <= 30. 



Since n <= 30 is true and we don't know population standard deviation, we use the t distribution.



The degrees of freedom would be n-1 = 25-1 = 24. 



So the answer is <font color="red">d. T distribution with 24 degrees of freedom </font>