Which of the following is the main function of an ID selector in CSS?

The main function of an ID selector in CSS is to apply styles to a single, unique element. This is because ID selectors are defined with a hash symbol (#) followed by the unique identifier. Each ID must be unique within a document, allowing the developer to target one specific element without affecting others.

Using an ID selector enables developers to set styles that are specific to that element, which can be useful for elements that require distinct styling that does not apply to any other elements on the page. This capability is particularly beneficial for JavaScript interactions and for applying styles that are not meant to be reused multiple times throughout a webpage.

Other options in the question relate to different aspects of CSS selectors. For instance, applying styles to several elements at once is typically accomplished using class selectors or element selectors, allowing for broader application of styles across multiple elements. The concept of applying a default style generally refers to styles set on the body or global level, not specific ID selectors. Lastly, while ID selectors can override class styles due to their specificity, the primary role of an ID selector is to provide unique styling to a designated element.