ANS:B - 6 V

@All.

I'm guessing that the frequency here is infinite because it's an instantaneous change, giving you a theoretically infinite resistance in the inductor (2*π*f*L), so approximately all of the voltage drop is in the inductor at t=0.

To break it down, a non-changing voltage level (constant DC voltage) has a frequency of 0, while an instantaneously changing voltage (such as a sudden flick of a switch) results to an infinite frequency (or extremely high frequency).

Then with impedances, we can infer that inductors resist high frequencies based on the formula while capacitors resist low frequencies more.

So, theoretically infinite frequency, with an inductor component in series, would result in a formula for the voltage drop of

V(inductor)=V(source)*2*π*frequency*inductance/(2*π*frequency*inductance+resistance)

With the voltage drop in the resistance to virtually zero.