which is true of a step - down transformer but not a step - up transformer?\nthe core is made of copper and…

Brief Explanations:

In a step - down transformer, the voltage is decreased from the primary to the secondary side. According to the transformer voltage - turns ratio formula $\frac{V_p}{V_s}=\frac{N_p}{N_s}$ (where $V_p$ and $V_s$ are primary and secondary voltages, and $N_p$ and $N_s$ are primary and secondary turns), for a step - down transformer $V_p>V_s$, so $N_p > N_s$, meaning the primary winding has more coils than the secondary winding. In a step - up transformer, $V_p<V_s$ and $N_p<N_s$. Also, the core of a transformer is made of iron to enhance the magnetic flux and the coils are made of copper for its good conductivity.

Answer:

D. The primary winding has more coils than the secondary winding.