Question

Why aren’t voluntary response samples usually representative of the population ?

Answer 1

The second choice is correct. An example of this is a voluntary "product feedback surveys" which tend to biased toward the negative. While people with both positive and negative feedback respond overall, customers who have a complaint are more motivated to take the time, and thus are overrepresented.

Answer 2

definitely the second, the people who volunteer usually have a motive to do so and will be the only ones represented.