Question

If a circuit contains 12V battery, a switch and a light bulb, what is the potential difference across the bulb ?

Answer 1

if this is your circuit then as soon as switch is closed there will be a potential drop of 12V across the bulb and if switch is open then there is no potential drop across the bulb at all,

Answer 2

and it will be 12V across bulb because after closing switch you can see its a parallel combination, so in parallel combination voltage across each element remains same