Which CSS property is used to make an element transparent?

The CSS property that is used to make an element transparent is opacity. The opacity property allows you to set the transparency level of an element by assigning it a value between 0 and 1, where 0 means fully transparent and 1 means fully opaque. This property affects not only the background but also the content inside the element, making it versatile for various design applications.

For instance, if you set an element's opacity to 0.5, it will appear semi-transparent, allowing the elements behind it to be seen through it. This can be particularly useful for creating layered designs or hover effects where you want to show or emphasize content while still revealing some of the background.

In contrast, visibility and display are properties that deal with the visibility and layout of elements rather than their transparency. Background-color simply changes the background color of an element but does not affect its opacity or transparency. Thus, opacity is the correct choice for controlling how transparent an element appears on the web page.