Which of the following is the proper syntax for a pseudo-class?

The correct syntax for a pseudo-class in CSS is indeed represented by the option that specifies the selector followed directly by a colon and the pseudo-class name. This involves using a regular selector (such as an HTML element, class, or ID) followed by a colon and the specific pseudo-class name, then defining the property and its value within curly braces.

For instance, if you want to style a link when it is hovered over, you would write something like a:hover { color: red; }. Here, a is the selector and :hover is the pseudo-class that denotes the specified style applies only when the user hovers over the link.

The other options presented do not correctly follow the syntax rules of CSS. The first option uses a non-standard format with an unnecessary period before the pseudo-class. The second option employs an incorrect operator (->) which isn't used in CSS for defining styles. The last option incorrectly uses a double colon (::); while double colons are valid for certain pseudo-elements in CSS, pseudo-classes should always be prefixed by a single colon (:). This distinction is important, as it helps differentiate between pseudo-classes (which apply to existing elements based on their state or position