Which CSS property is primarily used for adding background color to an element?

The CSS property primarily used for adding background color to an element is "background-color." This property allows developers to set a specific color behind the content of an element, which can enhance the visual appeal of a web page. By specifying the color value in various formats (such as named colors, RGB, RGBA, HEX, or HSL), designers can create a desired aesthetic or branding effect for their webpage.

Additionally, the other options do not serve the purpose of setting a background color. The "color" property is used to define the text color of an element rather than its background. The "fill" property, on the other hand, is relevant in SVG graphics for coloring shapes and does not apply to standard HTML elements. Finally, "bg-color" is not a valid CSS property for styling; it may stem from older coding practices or misunderstandings, but it is not recognized in current CSS standards. Thus, "background-color" is the clear and correct choice for adding a background color to an element in web development.