According to the Information Technology (Intermediary Guidelines and Digital Media Ethics Code) Rules, 2021, intermediaries that enable online interaction between users and have over 50 lakh registered users are classified as 'significant social media intermediaries'. These platforms, which include popular social networking services such as WhatsApp, Facebook, Instagram, and Twitter, are required to comply with additional regulatory requirements. These requirements are designed to ensure that such platforms exercise a certain level of vigilance and responsibility in handling user-generated content and interactions.
The correct answer is that these platforms need to observe "Due Diligence Requirements". This involves a series of obligations including, but not limited to, setting up grievance redressal mechanisms, removing content flagged for legality issues, ensuring accountability in content moderation, and cooperating with law enforcement in matters of national security and compliance with the law.
Therefore, given the options provided, the correct classification of the requirements that significant social media intermediaries must adhere to is "Due Diligence Requirements".