You transfer 20kJ20kJ of heat to a 4kg4kg block of iron and measure the temperature change of the iron. You apply 10kJ10kJ of heat to a second block of iron and find the temperature changes the same amount as for the first block. What is the mass of the second block

Ohm's law tells you that i = v/r where v is equivalent voltage and r is equivalent resistance of the circuit
since all circuits have the same voltage, your question is really asking which circuit has the lowest total resistance, since the lower the resistance, the higher the current
circuit a has a resistance of 5 ohms; circuit c is in series so has a resistance of 10 ohms; circuit b is in parallel and therefore has a resistance of 2.5 ohms (review your notes on adding resistances in parallel and series); therefore b has the highest current