Final answer:
Members of Congress primarily use opinion polls to learn about public opinion, as these offer a systematic way to gauge public sentiment.
Step-by-step explanation:
Members of Congress most often use opinion polls for learning about public opinion. Opinion polls are frequently cited by politicians to justify their positions on policies, and candidates use them strategically during campaigns. Surveys have evolved into precise tools through careful sampling and improved techniques, providing a scientific way of identifying and measuring public opinions. While other methods like social media, town hall meetings, and direct mail surveys are also utilized, opinion polls remain the primary, more systematized way members of Congress gauge public sentiment.