To calculate the velocity of an object, the *blank* of the position vs time graph should be calculated.

I think it would be DISTANCE

The correct answer is  "c".  it is "c" because melting is an endothermic change, and endothermic get's energy from surroundings. 

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