Answer:
С. .
This method of sampling is unbiased.
Explanations:
A method of sampling is referred to as bias when there is an imbalance in the sampling probability among members of the total population.
In this example, we have a survey sampling that involves anonymous members, there's no way the sampling probability can be lowered for some members than others because members of the whole population are unknown.
This is a fair sampling method and it is unbiased.