Which of the following is an example of a security violation?

The example of a security violation given in the question refers specifically to the accidental removal of classification markings. This act compromises the integrity of classified information and can lead to unauthorized disclosure, which is a fundamental breach of security protocols. Classification markings are critical for indicating the sensitivity of information and ensuring that personnel handle it appropriately according to its classification level. Removing such markings, even unintentionally, undermines the protective measures in place and may result in serious repercussions, including legal consequences and potential threats to national security.

The other options, such as attending training sessions, regular attendance of security briefings, and proper reporting of foreign contacts, are actions that contribute positively to maintaining security and compliance with established protocols. Engaging in these activities demonstrates adherence to security standards and a commitment to protecting sensitive information, contrasting sharply with the nature of a security violation.