The most effective way to mitigate the risk of unintentional data disclosure through the use of social media sites is to conduct user awareness training. User awareness training is a process of educating and informing the users about the security policies, procedures, and practices that are relevant and applicable to their roles and responsibilities. User awareness training can help to increase the knowledge, understanding, and compliance of the users regarding the data protection and privacy requirements, and the potential risks and consequences of data disclosure through social media sites. User awareness training can also help to influence the behavior, attitude, and culture of the users toward data security and privacy. The other options are not as effective as conducting user awareness training, as they are related to the technical, procedural, or contractual measures to mitigate the risk, not the human or behavioral measures to mitigate the risk. References = Risk and Information Systems Control Study Manual, Chapter 3: IT Risk Response, Section 3.3: IT Risk Response Implementation, page 145.