A current of 14.5 mA is maintained in a single circular loop with a circumference of 2.45 m. A magnetic field of 0.750 T is directed parallel to the plane of the loop. What is the magnitude of the torque exerted by the magnetic field on the loop

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