Which configuration should a network engineer apply to control protocol communication between subnets?

The correct choice for controlling protocol communication between subnets is the implementation of Access Control Lists (ACLs). ACLs are crucial for defining which traffic is permitted or denied access across a network, particularly at the boundaries of subnets. When a network engineer applies ACLs, they can specify rules that limit or allow communication based on IP addresses, protocol types, and ports. This is essential for managing network security, as it ensures that only authorized traffic can pass between different segments of the network, enhancing overall security posture.

Using VLAN Configuration is related to creating virtual local area networks to segment traffic within the same physical network but does not directly control the communication between different subnets. Routing protocols, while important for determining the best path for traffic between subnets, do not restrict communication; they facilitate it instead. Adjusting the subnet mask is primarily concerned with designing and organizing IP address space rather than controlling traffic flow, as it defines the size of each subnet but does not inherently restrict or allow communication. Therefore, Access Control Lists are the most effective tool in this scenario for managing and controlling inter-subnet communication.