Final answer:
The HTML document returned by a web server is contained in the payload of a network frame.
Step-by-step explanation:
The part of a network frame that contains the HTML document returned by a web server in response to a page request is the a) Payload. The payload is the part of the frame where the actual data is contained, including any HTML content. The other parts of the frame, such as the header and the trailer, contain control information such as source and destination addresses, error-checking codes, and other protocol-specific metadata. The data-link layer is responsible for the node-to-node deliverance of the message.