a temperature rise of 20 degree Celsius causes mecury to expand and rise 8cm in a thermometer tube. if the tube cross sectional area is doubles, the mecury will rise by cm?

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

Fb is the magnetic force  q is the chargev is the velocity of the charge  b is the magnetic field  the equation is: f=|q|vbsin0