What is the pressure of the system when all the stopcocks are opened, assuming that the temperature remains constant? (we can neglect the volume of the capillary tubing connecting the bulbs.)

Rain is pulled by gravity which is a non contact force

You will hear a hissing sound because of the carbon dioxide escaping the can or bottle

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