WAN (Wide Area Network) is a network that covers a broad area, connecting multiple LANs that might be on opposite sides of the world. It can span cities, states, or even countries. The internet is the largest example of a WAN.

The correct option is "Switch." Using a switch instead of a hub can help reduce collision domains in an Ethernet network. A switch is an intelligent device that forwards data only to the specific port where the destination device is located, reducing unnecessary traffic and collisions.

The Parity Bit is a basic error detection method that involves adding an extra bit to data to make the number of ones either even or odd. By checking this bit at the receiving end, errors can be detected in the transmitted data.

A Hub operates at the physical layer of the OSI model and broadcasts data to all devices in the network. It does not have intelligence to direct data only to the intended recipient, which can result in data privacy concerns.

Sliding Window is a flow control mechanism that allows a receiver to handle multiple frames before sending an acknowledgment. It helps optimize network efficiency by allowing the sender to transmit several frames before waiting for acknowledgments.

A Firewall is a network security device or software that monitors and controls incoming and outgoing network traffic. It enforces a set of security rules to determine whether to allow or block data packets, thus protecting a network from unauthorized access or potential threats.

The sliding window protocol handles situations when the receiver's buffer is almost full by reducing the window size. This is done to prevent overwhelming the receiver with data it cannot handle, ensuring smooth and efficient data transfer.

HTTP (Hypertext Transfer Protocol) uses port 80 by default. It is the protocol used for transmitting web pages and data on the World Wide Web. When you enter a website's URL without specifying a port, your browser assumes port 80 for HTTP.

OSPF (Open Shortest Path First) belongs to the Link-State class of routing protocols. In Link-State routing, routers exchange information about the state of their links, and each router builds a database that contains a complete map of the network's topology. This information is used to calculate the shortest path.

Adding CRC for error detection is a common practice. For critical data, combining CRC for detection with Reed-Solomon Code for error correction is a robust approach, as CRC helps identify errors and Reed-Solomon corrects them.