Calculate the speed of a proton that is accelerated from rest through an electric potential difference of 164 v.

Work done in moving a proton = potential difference×Charge of a proton
                                                 = 164×1.6×10⁻¹⁹ = 2.624×10⁻¹⁷ J
This work done should be equal to change in kinetic energy.
Initial speed of proton is zero therefore K.E initial will be zero.
Work done = final kinetic energy = 2.624×10⁻¹⁷ J
K.E = mv²/2
v² = 2(2.624×10⁻¹⁷)/1.6×10⁻²⁷ = 3.28×10¹⁰ m/s
∴ v = 1.811×10⁵ m/s 

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