Let's go through each answer individually to think about bias.
A. A survey is sent to all of the customers who are on the pharmacy's frequent buyer list.
Yes, this answer is biased, as the pharmacy is only taking into account a certain type of customer, therefore altering their results.
B. One hundred people from the pharmacy's customer list are selected at random, called on the phone, and asked.
No, this answer is not biased, as the customers who are surveyed are chosen completely at random.
C. Those customers who buy toothpaste with a coupon are selected, and the toothpaste brand they purchased is recorded.
Yes, this answer is definitely biased, for a number of reasons. First, the coupon causes discrepancies, as you don't know which toothpaste brands have coupons, the varying offers that come with these coupons, etc. Additionally, a customer that purchases their toothpaste with coupons is a select group of people, which doesn't allow the survey to get the most accurate results.
D. Every 10th customer who enters the pharmacy on one day is selected and asked.
No, this survey idea is not biased, (same as Answer B), because the people chosen for the survey are completely random, allowing for the survey to reach different types of customers and clarifying results.
Hope this helps! :)
