Final answer:
The sample in the opinion poll about movie-watching habits consists of the 831 people who responded to the survey question.
Step-by-step explanation:
The sample in this study is the group of individuals from whom data has been actually collected. In this case, the sample consists of the 831 people who responded to the question: "How many movies have you watched in a movie theater in the past 12 months?" Therefore, the correct answer would be option a. The 831 people who took the time to answer the question constitute the working sample for the poll.