The Presentation Layer (Layer 6) of the OSI model is responsible for the formatting, translation, and syntax of data for the application layer. Its primary functions include character code translation, data compression, and, most relevant to this question, data encryption and decryption. When User A uses PKI to encrypt an email message, the encryption process transforms the application data (the email content) into a secure format. This transformation is a classic Presentation Layer service, ensuring the data is protected before being passed down the stack for transmission and can be correctly decrypted and presented to User B's application.

A. Application: The application layer (e.g., the email client) generates the message content but delegates the task of data formatting and encryption to the Presentation Layer.
B. Transport: This layer secures the entire communication channel (e.g., using TLS) but does not perform encryption on the specific message data itself, as described in the scenario.
C. Session: This layer is responsible for establishing, managing, and terminating communication sessions between applications, not for handling the syntax or encryption of the data.

1. Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 1, Section 1.5.1, "The OSI Model," the text states, "The presentation layer is to provide services that allow communicating applications to interpret the meaning of the data exchanged. These services include data compression and data encryption."

2. Stallings, W. (2017). Data and Computer Communications (10th ed.). Pearson. In Chapter 2, Section 2.2, "The OSI Model," the Presentation Layer is defined as being responsible for formatting and code conversion, with encryption being a key service provided at this layer.

3. ISO/IEC 7498-1:1994, "Information technology -- Open Systems Interconnection -- Basic Reference Model: The Basic Model". Section 7.2.1, "Purpose," describes the Presentation Layer's role in managing the representation of information, which includes applying transformations like encryption to the data syntax used by applications.

4. Comer, D. E. (2018). Computer Networks and Internets (6th ed.). Pearson. In Chapter 11, "Network Layering Principles And The Osi 7-layer Model," the text specifies that the Presentation Layer handles data representation issues, explicitly listing encryption as one of its functions.