Why Ohm's Law rearranged for resistance is R = E / I, voltage divided by current. Here 12 volts divided by 4 amperes gives 3 ohms. A quick sanity check: with only 3 ohms of opposition, a fairly large current flows from a modest voltage, which matches the 4 amperes given.
Watch out The 48 ohm choice comes from multiplying 12 by 4, which actually gives power in watts (P = E x I), not resistance. The 16 and 8 ohm answers come from adding or subtracting the two numbers, which no Ohm's Law form does.
Resistance question means divide: volts over amps. If you multiplied, you found watts instead.