Step 1 :The question is asking which sampling distribution should be used given the information provided.
Step 2 :The sample size is less than 30, the population is normally distributed, and the population standard deviation is unknown.
Step 3 :Therefore, we should use a t-sampling distribution because the population is normal, and the standard deviation is unknown.
Step 4 :This is because the t-distribution is typically used when the sample size is small and the population standard deviation is unknown.
Step 5 :Final Answer: \boxed{\text{D. Use a t-sampling distribution because the population is normal, and } \sigma \text{ is unknown.}}