The Channel Tunnel connects which of the following pair of cities?

The correct option is London-Paris.

The Channel Tunnel (often referred to as the Chunnel) is a 50.46-kilometer (31.35-mile) underwater railway tunnel that runs beneath the English Channel at the Strait of Dover.
It connects Folkestone in Kent, United Kingdom, with Coquelles in Pas-de-Calais, northern France.

By connecting the railway networks of Great Britain and mainland Europe, the tunnel enables high-speed passenger trains (such as the Eurostar) to directly connect major cities on either side.
Specifically, the primary and most famous city-to-city rail connection facilitated by this tunnel is between London (United Kingdom) and Paris (France).