Which property correctly adjusts the boldness of text?

The property that correctly adjusts the boldness of text is the font-weight property. This CSS property allows you to specify the thickness of the font characters. Values for font-weight include keywords such as "normal," "bold," "bolder," and "lighter," as well as numerical values ranging from 100 to 900, where higher numbers represent bolder text. By using the font-weight property, designers can emphasize certain parts of the text, making it stand out in comparison to other text elements.

While other properties mentioned, such as font-size, affect the overall size of the text, and color alters its visual color, they do not influence the boldness of the text. The text-style property, which may imply italicization or other styles, is also not relevant to boldness adjustments. Thus, font-weight is the key property for changing the boldness of text in web design and styling.