Which of the Following Describes What the Property of Unbiasedness

Which of the following describes what the property of unbiasedness means?

A) The shape of the sampling distribution is approximately normally distributed.
B) The sampling distribution in question has the smallest variation of all possible sampling distributions.
C) The center of the sampling distribution is found at the population standard deviation.
D) The center of the sampling distribution is found at the population parameter that is being estimated.