Assigning a built-in role is the best option because it provides a predefined set of permissions targeting specific Azure resources, which in this case are network-related resources. The 'Network Contributor' role grants the ability to manage networking resources without granting access to other resources outside of this scope, aligning with the principle of least privilege. The other options are incorrect because they either provide permissions that are too broad, like the 'Contributor' role, which allows managing all resources, or are too restrictive, like the 'Reader' role, which only allows viewing resources without the ability to manage them. The 'Owner' role provides full access including the ability to delegate access, which is more than what is required for the network engineers.