What must periodicals contain according to the content requirement?

The correct answer highlights the requirement that periodicals must contain at least 25% non-advertising content. This standard ensures that periodicals offer substantial editorial substance alongside advertisements, maintaining a balance between information and promotional material.

This requirement is crucial because it helps to establish the integrity and value of the periodical as a source of information for readers. By mandating a minimum percentage of non-advertising content, regulators aim to prevent publications from becoming overly commercialized, thereby safeguarding the interests of subscribers and providing them with meaningful content.

Understanding this stipulation is key for both publishers and consumers, as it emphasizes the importance of maintaining quality in periodical publications.