The correct answer is:

the mean number of siblings for the randomly selected students

This is because the "statistic" refers to a value that is calculated from a sample, which in this case is the mean number of siblings for the 75 randomly selected students.

Would you like further clarification on this? Here are five questions related to this concept:

1. What is the difference between a statistic and a parameter?
2. Why is random sampling important in statistical studies?
3. How can Anna ensure that her sample is representative of the entire school?
4. What is the formula for calculating the mean in a sample?
5. How could Anna use this sample statistic to make an inference about the whole school?

Tip: When calculating statistics from a sample, the goal is often to estimate a population parameter, but the accuracy depends on the sampling method and sample size.