Which of the following describes the content of a message within a campaign?

The content of a message within a campaign is fundamentally tied to its purpose, which is to effectively communicate with the target audience and drive them towards a specific action. This is best captured in the choice that states it represents the campaign's purpose and call to action. In a typical marketing campaign, the message serves to not only inform recipients about what the campaign entails but also guides them on what they should do next, whether that's making a purchase, signing up, or engaging with the brand in some way.

The other options do not accurately reflect the nature of campaign messaging. Internal communication is not the focus of most marketing campaigns, which are aimed at external audiences. While product inventory can be a component of campaign content, it is not the core of a campaign's message. Summarizing customer service inquiries falls outside the scope of campaign messages, as these messages are about promotion and engagement rather than addressing customer service matters. Thus, the correct choice emphasizes the marketing intent and action-oriented nature of campaign content.