Estimate the thickness of the Earth's atmosphere. From your chemistry class, you know that the most common speed of a nitrogen molecule is 650 m/s. The molecules' velocity is pointed along random directions, but some molecules head straight up; i. e. directly away from the Earth's center.

Tons?

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