According to the WP29's "Guidelines on Personal data breach notification under Regulation 2016/679'', the communication of a personal data breach to the data subjects should be clear, concise, transparent, easily accessible and understandable, and use clear and plain language. The communication should also be made as soon as reasonably feasible and in close cooperation with the supervisory authority. The guidelines provide some examples of methods that may be effective for communicating a breach to data subjects, such as a direct electronic message (e.g. email, SMS, direct message), a postal notification, a prominent advertisement in print media, or a notice on the homepage of the affected website. However, the guidelines also state that a notice on a corporate blog or social media would not be an effective method of communication, as it would not reach all the affected data subjects and would not allow them to take immediate action to protect themselves. Therefore, the correct answer is C. A notice on a corporate blog. Reference:
WP29's "Guidelines on Personal data breach notification under Regulation 2016/679'', pages 20-211