ANS:D - 4

Duralumin is an aluminum alloy, not an alloy of aluminum and copper. It typically contains a small percentage of copper, along with other alloying elements such as magnesium and manganese. The percentage of copper in duralumin is approximately 4%. Duralumin was historically one of the earliest age-hardenable aluminum alloys and was widely used in aircraft construction during the early 20th century. Its composition varies depending on the specific application and desired properties, but typically, it contains around 4% copper. So, the correct answer is: 4