What type of service would a tunnel broker provide?

A tunnel broker provides IPv6 tunneling over IPv4, which allows users to connect to an IPv6 network even when their internet service provider only offers IPv4 connectivity. This is particularly important as the internet transitions from IPv4 to IPv6 due to the exhaustion of IPv4 addresses. By utilizing tunneling, a tunnel broker encapsulates IPv6 packets within IPv4 packets, enabling communication between IPv6 devices across an IPv4 infrastructure. This service is crucial for users and organizations that need to access or transition to IPv6 while maintaining interoperability with existing IPv4 networks.

The other options, while they address different services related to networking and internet usage, do not pertain specifically to the core function of a tunnel broker. Secure file transfer focuses on safe data transmission methods, firewall protection is related to securing a network from unauthorized access, and email forwarding pertains to the management of email communications rather than the tunneling of protocols. Hence, the role of a tunnel broker distinctly aligns with providing IPv6 tunneling over IPv4.