Answer:
a) Any sample statistic used to approximate a population parameter.
Explanation:
An unbiased estimator is any sample statistic used to approximate a population parameter.
In mathematics, the bias of an estimator refers to the difference between the true value of the parameter being estimated and the estimator’s expected value.
If a statistic is not an underestimate or overestimate of the population parameter, then it is unbiased.
That is why an unbiased estimator is an accurate sample statistic used to approximate a population parameter.