D is correct - Application layer.
An open Modbus TCP/IP specification was developed in 1999. Modbus, a messaging protocol for client/server communication, operates at Layer 7 (the Application Layer) of the Open Systems Interconnection (OSI) model. Modbus is designed to facilitate communication between devices, such as sensors, motors, and PLCs (Programmable Logic Controllers), and supervisory systems, allowing for data exchange and control. www.modbus.org
The Modbus Application Protocol operates at the Application Layer of the OSI model. This layer (Layer 7) defines the rules for data formatting, communication, and interaction between applications, which is exactly where MBAP resides. MBAP is part of the Modbus TCP/IP implementation and specifies how messages are constructed and interpreted for communication
D. Application layer
The Modbus Application Protocol (MBAP) operates at the Application layer of the OSI model. The Application layer is responsible for providing network services directly to end-users or applications. In the case of Modbus, the Application layer defines the rules and structure for communication between devices on a network, specifying the format and content of messages exchanged between Modbus devices.
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