How would you change the font size of an element to 20 pixels?

The correct choice involves using the font-size: 20px; declaration. This is the standard CSS property used to adjust the size of the font within an HTML element, where "20px" specifies the size in pixels. CSS properties often follow a specific naming convention, and "font-size" is the officially recognized property for adjusting text size.

In contrast, options that suggest using "font-size: large," "size: 20px," and "text-size: 20px" would not work effectively for setting the font size to 20 pixels. While "large" is a valid value for the font-size property, it does not specify an exact size in pixels like the correct choice does. The other incorrect options feature non-existent properties or incorrect property names for defining font size in CSS.