Network-based email was initially exchanged on the

Network-based email was first exchanged on ARPANET, the early packet-switched network that preceded the internet.

SMTP is a later protocol for transporting mail, not the network on which email first appeared.

TCP/IP is the protocol suite that underlies the internet, again a protocol rather than that original network.

No other choice fits because ARPANET is the correct historical answer.

The first networked email was sent on ARPANET in 1971 by Ray Tomlinson.

ARPANET was funded by the United States Department of Defense's ARPA.