Explanation
The technique that would fulfill the requirement of transferring data in multiple networks is 802.1Q tagging.
802.1Q tagging is a method of adding a tag or identifier to Ethernet frames that indicate which VLAN (Virtual Local Area Network) they belong to. VLANs are logical subdivisions of a network that allow devices in different physical locations or segments to communicate as if they were in the same network. VLANs improve network performance, security, and management by reducing broadcast traffic, isolating sensitive data, and grouping devices by function or department. By using 802.1Q tagging, two Layer 2 switches can exchange data from multiple VLANs over a single trunk link, without mixing or losing the VLAN information. References: CompTIA Network+ N10-008 Certification Study Guide, page 64; The Official CompTIA Network+ Student Guide (Exam N10-008), page 2-12.

The type of data center architecture that will most likely be used in a large SDN and can be extended beyond the data center is spine and leaf. Spine and leaf is a network topology that consists of two layers of switches:
spine switches and leaf switches. Spine switches are interconnected to each other and form the core of the network, while leaf switches are connected to each spine switch and form the access layer of the network.
Spine and leaf topology provides high scalability, performance, and flexibility for data center networks, especially for SDN (Software Defined Networking) environments that require dynamic traffic flows and virtualization. References: CompTIA Network+ N10-008 Certification Study Guide, page 16; The Official CompTIA Network+ Student Guide (Exam N10-008), page 1-9.

The reason the network is not functional is that STP (Spanning Tree Protocol) is not configured on the switches. STP is a protocol that prevents loops in a network topology by blocking redundant links between switches. If STP is not enabled, the switches will forward broadcast frames endlessly, creating a broadcast storm that consumes network resources and disrupts communication. Reference: CompTIA Network+ N10-008 Certification Study Guide, page 67; The Official CompTIA Network+ Student Guide (Exam N10-008), page 2-14.