Too bad Schweser doesn't say anything about this not working for a bond not trading at par in the notes (at least I don't remember them making note of that).
I would've chosen A...
tax-free yield (muni)/(1-tax rate)
0.04/0.72 = 0.055555 ~= 5.56%
I guess it makes sense though, considering the bond is at a discount and you're trying to find the parity yield that makes them equal factoring in taxes? This is such a bullsh!t question that they've worded poorly.
If you can't find tax-equvalent yields for bonds trading at anything other than par, they should've said, can't be determined or something instead of giving the impression that it would produce a yield/percentage higher or lower than what is listed in the choices. I guess I'll have to be watching out for more tricks?