You updated the proper network configuration file on your Ubuntu host but changes did not take effect. What command should you run?

The command you should run to apply the updated network configuration on an Ubuntu host is to restart the networking service. The correct command, which is option B, leverages the service command to manage services on a Linux system.

Using sudo service networking restart effectively stops and then starts the networking service. This process allows the changes made in the configuration file to be loaded and applied. The sudo is necessary because restarting network services typically requires superuser privileges to ensure that the operation does not disrupt system functions.

Additionally, the use of the word "service" in this context refers to the management system prevalent in many Linux distributions, including Ubuntu, for handling system services. This is particularly relevant when updating network settings that might not automatically propagate upon saving the configuration file.

The other options either reference incorrect command structures or do not align with the proper syntax recognized within the Ubuntu system for managing services. Thus, option B stands out as the correct choice for reapplying network configurations correctly.