Which of the Following Accurately Describes a Sampling Distribution of the Mean

Which of the following accurately describes a sampling distribution of the mean?

A) It is the mean of any one sample from a group of related populations of different sizes.
B) It is the mean distribution of any one sample from a group of samples of varying sizes from a population.
C) It is the mean of half the samples from a related group of populations.
D) It is the distribution of the means of all possible samples of fixed size n from a population.