Question

in order to make an electric current flow, it is necessary to have
a copper conductors
b an ammeter
c a difference of potential
d an open circuit

Explanation:

Electric current flows due to a potential difference (voltage) between two points in a circuit. Copper conductors are just a type of material that can carry current, an ammeter measures current, and an open - circuit does not allow current to flow continuously.

Answer:

C. a difference of potential