The student then released the cart from a lower position such that the cart only travels 95 cm. What was the magnitude of the carts acceleration on release 2?

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.

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