Suppose you want to estimate the average number of sick days taken by all employees at a large company during a year. Suppose (unknown to you) this is four days. You devise a way to randomly select 200 employees for a survey and use the sample mean number of sick days as an estimate of the population mean for all workers at the company. Unknown to you, the sampling distribution of the sample mean has an average value of 4.8. This means that

a. your estimator (the sample mean) is not an unbiased estimator of the population mean because your sampling method tends to pick people that take more sick days.
b. your estimator is an unbiased estimator of the population mean.
c. if you survey more American adults, your sample mean will tend to get closer to the population mean.
d. the person computing it doesn't favor any particular outcome.