I would also say D. If a problem does not say "assuming a normal distribution" in its statement, then one can use the t-distribution instead for smaller sample sizes (less than 30).
Actually, the correct assumption made when finding a confidence interval for a population mean based on a sample of size 11 is that the sampling distribution of the sample means is approximately normal. This assumption is based on the Central Limit Theorem, which states that for a large enough sample size, the distribution of sample means will be approximately normal, regardless of the shape of the population distribution. So, the correct answer is C: "The sampling distribution of the sample means is approximately normal."
