What Cisco AirOS-based controller would you recommend to a customer who needs a physical controller...

The Cisco 3504 Wireless LAN Controller is a physical, AirOS-based controller designed for small to mid-sized enterprise networks. It supports up to 150 access points and is purpose-built for scalability in branch or mid-market deployments. While Mobility Express is limited in AP count and is not a dedicated controller, the 8540 is for large deployments. Cisco WLC is a generic term. Thus, option C is the most accurate recommendation.

—