Aproton moves at a speed of 2.0 x 10^7 m/s at right angles to a magnetic field that is directed into the page with a magnitude of 0.1t, calculate the radius of the orbit

The Lorentz force acting on the proton is equal to:

where
q is the proton charge
v is the proton speed
B is the intensity of the magnetic field
is the angle between the direction of v and B

since the proton travels perpendicularly to the magnetic field, in this case , so the Lorentz force in this case is simply


The magnetic force provides also the centripetal force that keeps the proton in circular motion:

where 
m is the proton mas
r is the radius of the orbit

If we re-arrange this equation and we use the data of the problem, we can find the radius of the orbit:

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