Which of the following is a valid CSS rule?

The choice of "h1 { color: blue; }" represents a valid CSS rule because it follows the correct syntax for defining a style for a specific HTML element. In this case, "h1" specifies the header level one element, and the curly braces that come after indicate that the styles defined within them are designated for that element.

Within the curly braces, "color: blue;" is a declaration that tells the browser to render all h1 elements with the text color blue. This structure is essential in CSS, as it clearly associates a style with a particular HTML tag.

The other choices contain syntax errors or incorrect formatting. For instance, using an arrow in the second option is not a valid way to specify a CSS rule. The first option uses an invalid value for "font-size," since "large" is not a recognized value in modern CSS syntax, which requires specific units like "px," "em," or "rem." The last choice improperly formats the CSS declaration, separating the property ("color: blue;") from the selector ("h1") in a way that does not conform to standard CSS syntax.