The nominal exchange rate (NER) is given by:
NER = RER × (foreign price / US price)
    = 3/4 × ( 6 / 3)
    = 3/2
In fact, with 1$ you can buy 1/3 of a US gallon, which corresponds to 1/4 of a foreigner gallon, which you would pay 3/2units.
Hence, the correct answer is a)Â 3/2 units of country A's currency per dollar.