BBC Two originally aired Dragon's Den when the British programme launched in 2005.
The programme premiered on BBC Two as a factual-entertainment series centred on entrepreneurs seeking investment. BBC Two was the channel associated with its launch and early identity before the programme later became part of BBC One's schedule.
The channel helped position Dragon's Den alongside other factual, business and reality formats aimed at audiences interested in how companies are built and financed. The programme's success eventually gave it a larger mainstream profile within the BBC.
BBC One is the most common alternative answer because it later broadcast the series and is the BBC's principal general-interest channel. That later association should not be projected backward: the launch broadcaster was BBC Two.