How can density be used to identify a substance?

Each element has its own specific density. Density is a physical characteristic, it could be used to identify unknown substance because of its uniqueness. No two elements have identical densities, and densities of the same substance may it be of different volume has closely identifiable densities.

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