Which is an example of a positive feedback loop? i. warmer temperatures on earth's surface decrease the evaporation of water. ii. water evaporation creates low-altitude clouds that reflect sunlight back into clouds. iii. water evaporation creates high-altitude clouds which absorb terrestrial energy that would otherwise escape the atmosphere?

It would be "Warmer temperatures on earth's surface decrease the evaporation of water."

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