The motion of a 3.0 kg mass on the end of a spring is described by
x(t) = (0.20 m)*Cos(2t/3.0s).
What is a time when the system is at the equilibrium position?

Please explain how to find this answer

Answer(b) 60 miles per hour;

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