Salford is the British city where the BBC’s MediaCityUK complex is located and associated with later production of Dragon's Den.
MediaCityUK is a major media hub at Salford Quays in Greater Manchester. The BBC moved substantial operations there, and the site became associated with production of several well-known programmes, including later Dragon's Den activity.
Manchester is a common wrong answer because Salford and Manchester are neighbouring cities within the same metropolitan area. MediaCityUK is specifically in Salford, across the River Irwell from central Manchester. Liverpool and Leeds are also major northern media and business centres, but they are not the city named by this location.
The question concerns the complex’s city, not the wider county or metropolitan region. The precise answer is therefore Salford, while Greater Manchester is the broader ceremonial and metropolitan context.