The correct answer is 'Encrypted email with a pre-shared key or digital certificates' because it provides a level of security suitable for transmitting sensitive information by encrypting the content of the email, thus ensuring that only authorized recipients with the correct keys or certificates can decrypt and access the information. In contrast, the incorrect answers either lack sufficient security measures to protect sensitive information, such as standard email lacking encryption, or are not practical for the distribution of reports, such as secure instant messaging which is typically not used for the distribution of extensive documents like penetration testing reports.