What must be established for transactions to occur between channels?

For transactions to occur between channels in a blockchain network, establishing an open layer of communication is essential. This communication layer allows different channels to share information and conduct transactions seamlessly.

In a multi-channel architecture, such as those found in IBM Blockchain, channels can represent different groups or communities that have their own subset of transactions and privacy requirements. Having an open layer means that these channels can effectively communicate and exchange necessary information, which is crucial for the integrity and functionality of the network. It supports inter-channel transactions, allowing the blockchain to operate as a cohesive unit while maintaining the distinctiveness of each channel's operations.

Redundant network backups ensure data reliability, strict security protocols protect against unauthorized access, and centralized control mechanisms can help in governance, but none of these aspects directly facilitate communication needed for transactions between channels. An open layer of communication specifically addresses the necessity of interaction between these varied components of the blockchain ecosystem, making it the correct answer.