What is the magnitude of the kinetic frictional force

Magnitude of kinetic force is the product of the normal force and the coefficient of kinetic friction.

Explanation:

The kinetic frictional force is the force which is experienced in the moving surfaces of bodies.The magnitude of the force is affected by the coefficient of kinetic friction between the material of two bodies in contact. Mathematically, it is expressed as;

Fk=μk*η

where;

Fk=force of kinetic frictionμk=coefficient of kinetic frictionη=normal force

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