Final answer:
Feedback is the tool that allows advertisers to monitor how effective their advertising messages are by analyzing the audience's reactions to the message.
Step-by-step explanation:
The component that provides the sender of an advertising message with a way of monitoring the effectiveness of the message is B. Feedback. Feedback refers to the responses or reactions of the audience towards the advertising message. It allows advertisers to measure the impact of their message and assess how well it is being received by the target audience. Techniques to garner feedback can include tracking sales figures pre- and post-advertisement, soliciting customer reviews, or using digital analytics in the case of online advertising, such as click-through rates or engagement metrics.