Network Layer:
The network layer, also known as the third layer of the OSI model, is responsible for the routing and forwarding of data packets across different networks. It provides end-to-end communication between hosts on different networks by establishing logical connections and determining the most efficient path for data transmission.

Option A: Internet Protocol (IP)-ARPANET:
- The Internet Protocol (IP) is a network layer protocol that enables communication between devices connected to different networks.
- ARPANET was the precursor to the modern internet and used IP as its network layer protocol.
- This option is correct because IP-ARPANET represents the use of the network layer protocol (IP) in the historical context of ARPANET.

Option B: X.25 Packet Level Protocol (PLP)-ISO:
- X.25 is a packet-switched network protocol suite that was widely used in the 1980s and 1990s, primarily for connecting remote devices to mainframes over public networks.
- The Packet Level Protocol (PLP) is a component of the X.25 protocol suite that operates at the network layer.
- ISO (International Organization for Standardization) is a standard-setting body that has defined various protocols, including the X.25 protocol suite.
- This option is correct because X.25 PLP is an example of a network layer protocol defined by ISO.

Option C: Source Routing and Domain Naming-USENET:
- Source routing is a technique where the sender specifies the complete route that a packet should take through the network.
- Domain naming is the process of assigning meaningful names to network resources, such as computers and services.
- USENET is a distributed discussion system that was popular in the early days of the internet.
- While source routing and domain naming are relevant concepts in networking, they are not specific examples of network layer protocols.
- This option is incorrect because it does not represent examples of network layer protocols.

Option D: All of these:
- This option is correct because it includes both IP-ARPANET and X.25 PLP, which are examples of network layer protocols.
- It is important to note that this option does not include source routing and domain naming, as they are not network layer protocols.
- Therefore, the correct answer is option D, "All of these."