The correct answer is A: by using the style attribute within another HTML element.
Inline CSS can be applied directly to an HTML element by using the style
attribute and specifying the CSS properties and values within that attribute. For example:
<p style="color: red; font-size: 20px;">This is a red paragraph with a font size of 20px.</p>
Options B and C refer to other methods of adding CSS, such as using an external stylesheet or importing a stylesheet, which are not inline styles.