In CSS, how do you select an element with the id "header"?

To select an element with the id "header" in CSS, the correct syntax is to use a hash symbol (#) followed by the id name. This is because the id selector in CSS is specifically denoted by the hash symbol, which tells the browser to apply styles only to the HTML element that has that specific id. For example, if you want to set the background color of an element with the id "header," you would write:

#header {

background-color: blue;

}

In this case, only the element with the id "header" will be affected by the styles defined in this rule. The other options presented do not correctly represent how to select an id in CSS. There are other selector types in CSS, such as the universal selector (*) or class selectors (.), but they serve different purposes and cannot be used to effectively target an id. The correct use of the id selector ensures that styles are accurately applied to the intended element.