The original British series Dragon's Den began airing on BBC Two.
The programme premiered on BBC Two in 2005, placing business pitches and investor negotiations in the channel's factual and entertainment schedule. Its straightforward format helped turn discussions about valuation, ownership and entrepreneurship into accessible television. The series later became one of the BBC's best-known business programmes.
BBC One is a plausible but incorrect answer because the show has reached a broad mainstream audience and the BBC has used different channels for major programmes over time. The channel at the beginning of the British run, however, was BBC Two. BBC Three and BBC Four were also BBC channels, but neither was the original home of the programme.
Channel information can be confused with broadcast changes in later years. A programme's original channel is a historical fact about its launch, whereas later scheduling decisions describe a different stage of its television history. The question specifically asks where the British series began airing.