What framework tool can you use to quickly send response messages back to the user?

The Response Builder is the correct choice for quickly sending response messages back to the user. This tool is specifically designed to facilitate the construction of responses in a streamlined manner. It allows developers to create messages with various formats and structured data efficiently, which can be particularly useful in scenarios where rapid communication with users is critical. By using the Response Builder, you can ensure that messages are not only sent promptly but also formatted correctly according to the requirements of the user interface and the communication channel being used.

The other options may have roles in the overall architecture of a conversational application but do not focus solely on the quick transmission of responses. The Dialog Manager, for example, handles the flow of conversation but isn't specifically tailored for composing and sending responses. The Activity Handler processes incoming activities but may require more configuration to respond effectively. The Message Composer, on the other hand, could be involved in message formatting but is not a standard tool specifically recognized within Azure for quick response generation. Thus, Response Builder stands out as the best tool for achieving swift communication with users.