Which of the following is the MOST secure method for sending the penetration test report to the client?

Hosting the report on an online storage system introduces third-party risk and does not guarantee the highest level of confidentiality without explicit and strong encryption controls.

A password-protected ZIP file's security depends on the password strength and secure out-of-band communication of that password, which is often difficult to achieve reliably.

Transferring via webmail using an HTTPS connection only encrypts the data in transit; the report itself might remain unencrypted on mail servers, and email is generally not considered a sufficiently secure medium for highly confidential penetration test reports.

Encrypting the report with the client's public key (e.g., using PGP/GPG) ensures that only the client, possessing the corresponding private key, can decrypt and read the highly sensitive information, providing strong end-to-end confidentiality during transmission and storage. This is considered the most secure method for sending confidential documents digitally.