Hello there. To solve this question, we'll analyze each of the options and determine why it is the case for this sample to be biased.
We know that this local maganize is available by subscription and at newsstands and they asked the readers by mailing them comment cards for the subscribers.
A. The magazine changed its content for the issue including the comment card.
Since the question didn't mention this fact, we cannot assume this to be a correct option, we only know they sent comment card to the readers.
B. Very few comment cards were returned
The question says that 97% of the readers were extremely satisfied with the content of the magazine. But we know that these readers that received a comment card were subscribers.
This means that, considering each subscriber sent its comment card back, 3% of them were not satisfied, but we don't know how many subscribers this magazine has.
Hence this answer is not correct as well.
C. The survey only considered subscribers, and not those who purchase the magazine at newsstands.
This might be the answer, considering that the comment card were only mailed to the subscribers and, since we don't know the proportion between subscribers and those who buy at the newsstands, this sample is biased.
D. Not enough comment cards were distributed.
The question said that the magazine mailed a comment card for each of its subscribers, so we cannot take this option into consideration.