What technique did Mr. Curtain plan to use to expand the Whisperer's function?

The technique that Mr. Curtain planned to use to expand the Whisperer’s function involves allowing thoughts to be transmitted without his presence. This concept is central to Mr. Curtain's ambition as it takes the ability of the Whisperer beyond the limitations of being physically present, thus broadening its influence over the minds of children, irrespective of where they are located. By removing the need for direct contact, Mr. Curtain seeks to create a more pervasive control mechanism that can infiltrate children's thoughts at any time and from any location.

This plan emphasizes the effective and insidious nature of the Whisperer, which is designed to manipulate thoughts directly, thus posing a significant threat to free will. It showcases Curtain's ultimate goal of achieving total control over individuals’ thoughts and actions. The other choices, while relevant to Mr. Curtain's overall strategies, do not specifically address the technical enhancement of the Whisperer itself in the same way.