To answer this question, you need to understand the concept of complex elements.

Complex elements are elements in HTML that can contain other elements and/or text. They are made up of a start tag, an end tag, and the content in between. Empty elements, on the other hand, do not have any content between the start and end tags.

Let's go through each option to understand whether it is a kind of complex element or not:

Option A) Empty elements - This option is not a kind of complex element because empty elements do not have any content between the start and end tags. Examples of empty elements in HTML include <br>, <img>, and ``.

Option B) Elements that contain only other elements - This option is a kind of complex element because it contains other elements within it. For example, <div> is a complex element that can contain other elements such as <p> or <span>.

Option C) Elements that contain only text - This option is a kind of complex element because it contains text between the start and end tags. For example, <p> is a complex element that can contain text.

Option D) Elements that contain only numbers - This option is not a kind of complex element because elements in HTML cannot contain only numbers. Elements can only contain text, other elements, or a combination of both.

The correct answer is D. Elements that contain only numbers are not a kind of complex element in HTML.