-

A 5.00-kg body accelerates from 3.25 m/s to 9.75 m/s in 3.0 seconds when a force acts upon it. Find (a) the change in momentum of the body, (b) the impulse produced by the force, and (c) the magnitude of the force.

Independent variable is something that is changed in a experiment. 

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

Answermuch larger;