A Network Address Translation (NAT) gateway solely performs the function of translating private IP addresses into public IP addresses, which aligns with the company's requirements for the branch office. A Unified Threat Management (UTM) device, while capable of performing NAT, includes multiple other security features that the scenario deems unnecessary, adding complexity. An IDS/IPS focuses on intrusion detection and prevention and does not handle address translation. A load balancer is primarily used for distributing network or application traffic across multiple servers, which is irrelevant to the task of address translation.