The default gateway is the IP address of the router that connects a network to the internet or another network.
The default gateway is usually configured on the devices that need to access the internet or other networks, such as PCs, servers, or routers. If the router was configured with an incorrect default gateway, it would not be able to forward packets to the correct destination, and internet access would not be possible.
The other options are not the most likely causes of the issue. The Ethernet interface is the physical port that connects a device to a network using a cable. If the Ethernet interface was configured with an incorrect IP address, it would cause a problem with the local network connectivity, not the internet access. The loopback address is a special IP address that refers to the device itself, usually used for testing or troubleshooting purposes. If the router was configured with an incorrect loopback address, it would not affect the internet access, as the loopback address is not used for routing packets to other networks. The serial interface is another type of physical port that connects a device to a network using a serial cable, often used for WAN connections. If the serial interface was configured with the incorrect subnet mask, it would cause a problem with the WAN connectivity, not the internet access, as the subnet mask is used to determine the network and host portions of an IP address.
ReferencesWhat is a Default Gateway? | HowStuffWorksWhat is an Ethernet Interface? - Definition from TechopediaWhat is a Loopback Address? - Definition from TechopediaWhat is a Serial Interface? - Definition from Techopedia