User A is writing a sensitive email message to user B outside the local network. User A has chosen to use PKI to secure his message and ensure only user B can read the sensitive email. At what layer o

The Application layer (Layer 7) provides user-facing network services and protocols such as SMTP and IMAP, but does not itself perform the encryption or decryption of data payloads.

The Transport layer (Layer 4) is responsible for end-to-end communication reliability, segmentation, and flow control via protocols like TCP and UDP, not data encryption.

The Session layer (Layer 5) manages the establishment, maintenance, and termination of communication sessions between applications, and is not responsible for encrypting or decrypting data.

The Presentation layer (OSI Layer 6) handles data formatting, translation, compression, and encryption/decryption. When PKI is used to encrypt an email, the message is encrypted at this layer before being passed down the stack for transmission, and decrypted at this same layer on the recipient's side - ensuring only the holder of the correct private key can read the plaintext content.