6 Bella's mum has had the café _________ was a baby.

6 Bella's mum has had the café since Bella was a baby.

В данном предложении необходимо использовать предлог "since", так как указывается конкретная точка во времени (Bella was a baby).

Ответ: Bella's mum has had the café since Bella was a baby.