Connection-oriented:
TCP (Transmission Control Protocol) is a connection-oriented protocol. This means that before any data is sent between two devices, a connection must be established. This connection is established through a process called the "three-way handshake," where the client and server exchange a series of messages to agree on the parameters of the connection. Once the connection is established, data can be reliably transmitted between the devices.

Process-to-process:
TCP provides a process-to-process communication service. This means that it allows applications running on different devices to communicate with each other. TCP accomplishes this by using port numbers to identify specific processes or services on each device. When data is sent through TCP, it includes the source and destination port numbers, allowing the receiving device to deliver the data to the correct process.

Transport layer protocol:
TCP operates at the transport layer of the OSI model. The transport layer is responsible for ensuring reliable data delivery between two devices. TCP provides features such as error detection, flow control, and congestion control to ensure that data is transmitted accurately and efficiently. It breaks the data into smaller packets, sends them over the network, and reassembles them at the receiving end.

All of the mentioned:
The correct answer is option 'D' - All of the mentioned. TCP is indeed a connection-oriented protocol, it provides process-to-process communication, and it operates at the transport layer of the OSI model. These characteristics make TCP a widely used and essential protocol for reliable data transmission over IP networks.

In summary, TCP is a connection-oriented, process-to-process, transport layer protocol. It establishes a connection between devices, allows applications to communicate with each other, and ensures the reliable delivery of data.