 Using PGP (Pretty Good Privacy) encryption ensures that the report file is securely encrypted with the client's public key. Only the client can decrypt the file using their private key, ensuring confidentiality during transit.

 Details:

Option Analysis:

A. Creating a link on a cloud service and delivering it by email: This method is susceptible to interception or unauthorized access.

B. Asking for a PGP public key to encrypt the file: Provides end-to-end encryption ensuring that only the intended recipient can access the file.

C. Requiring FTPS security to download the file: While secure, it does not provide the same level of end-to-end encryption as PGP.

D. Copying the file on a USB drive and delivering it by postal mail: While physically secure, it is not practical and poses a risk of loss or theft.

 References: PGP encryption is a widely accepted method for securing sensitive data. It is recommended by many cybersecurity standards and best practice guides.