Cesar wants to ensure that his organization's SCADA and ICS devices remain secure. What is the most effective way to ensure that network attacks cannot impact his operation's critical infrastructure?

Physically isolating the network for the SCADA and ICS systems can prevent attacks from outside of the network. In critical infrastructure like power generation stations, this is a common technique to ensure that external attacks cannot cause outages to critical physical infrastructure. Separate VLANs is a common logical control, but attacks against the network or using VLAN hopping techniques can allow attackers to bypass the separation. TLS is useful for protecting traffic but does not meet the security requirement described, and SDN is useful for managing networks but again does not meet the enhanced security requirements in the question.