Which of the following is a valid CSS rule? h1 color: blue; ) style="color:blue;" css="color:blue;" h1 color ( blue; )

The correct answer is: color: blue;<br /><br />Explanation: A valid CSS rule consists of a selector and a declaration block. The selector specifies the HTML element to be styled, and the declaration block contains the property and value to apply to the selected element. In this case, 'h1' is the selector, and 'color: blue;' is the declaration block, making it a valid CSS rule.