If he wants to have a unbiased sample to study the preferred bowling hours, he has to take a sample that is independent of the hour the customers are bowling.
This discards sampling the customers who are in the lunchtime league or go to midnight bowling.
Also, the customers who bowl every tuesday can be a biased sample because the others days of the week can have a different distribution for the hours.
The only group that is unbiased from the list is the customers who are on their mailing list.
Answer: option D.
