The correct answer is A. ESSID. An ESSID (Extended Service Set Identifier) is used when multiple wireless access points are configured to present the same wireless network name (SSID) across a larger coverage area, such as an entire building or campus floor. This allows users to move from one location to another while remaining connected to what appears to be a single wireless network. In practice, this supports seamless roaming between APs when they are properly configured with the same SSID, compatible security settings, and coordinated channel planning.
The other options do not fit the requirement. Wireless mesh is a topology that allows APs to connect to one another wirelessly, but it does not by itself describe using one common WLAN name across all APs. Band steering pushes dual-band clients toward the 5 GHz or 6 GHz band for better performance; it is not about creating one building-wide WLAN name. WPA3 is a wireless security standard and relates to encryption and authentication, not SSID design.
The key phrase in the question is "a single wireless LAN name across the entire building", which points directly to an ESSID deployment.