If the merry-go-round makes one revolution in 10 seconds, what is the child’s linear speed?

The child's linear speed is
              
     (pi / 5) x (the child's distance from the center of the ride, in feet)

                                                                                        feet per second.

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