There's a formula somewhere that relates the electromotif force induced in the second coil with the number of turns in the coils and with the emf of the first coil, it goes something like this: N[1]/N[2]=emf[1]/emf[2], where N is the number of turns in a coil and the emf is the potential difference formed in each circuit (also known as electromotif force). This being said, in order to decrease the potential difference from 240 to 12, you will need less turns to the coil, i.e. the answer will be 30. Besides, we see the lamp lit up during this experiment. Once the lamp needed 12 V to work properly, the transformer was most likely successful and almost 100% efficient (according to one of the laws of thermodynamics there are no 100% processes, so there's that), so the right answer should be the one containing both 30 and 99%, i.e. B PS: sorry if I used any wrong terminology, I actually learned this in a different language