The correct answer is: D. Both (C) and (D)
A rule consists of a selector and a declaration.
A selector is a pattern that matches elements in the document. It can be a simple tag name, a class name, or an ID.
A declaration is a statement that specifies how an element should be displayed. It has a property name and a value.
For example, the following rule matches all elements with the class "red":
.red {
color: red;
}
This rule specifies that all elements with the class "red" should have a red color.
Here is another example:
```
header {
background-color: blue; } ```
This rule specifies that the element with the ID "header" should have a blue background color.
I hope this helps! Let me know if you have any other questions.