Suppose that a monopoly firm finds that its MR is $72 for the first unit sold each day, $71 for the second unit sold each day, $70 for the third unit sold each day, and so on. Further suppose that the first worker hired produces 5 units per day, the second 4 units per day, the third 3 units per day, and so on.

That would be the corect answer hope this

The relationship of both of them

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