In HTML, the rel attribute is used to define the relationship between the current document and the linked URL (the one specified in the href attribute). It is often used in anchor (<a>) tags, as well as in <link> and <area> tags.

Example:

In this example, the rel="noopener" attribute specifies that the relationship between the current page and the linked page is to open in a new window without passing any information about the current page.

Here’s what the other options mean:

• URL: There is no URL attribute in HTML for defining the relationship between documents.
• REV: This attribute was used historically to define the reverse relationship (the inverse of rel), but it is deprecated and rarely used today.
• All of these: Since only rel is the correct attribute for this purpose, "All of these" is incorrect.

So, the rel attribute is the one that defines the relationship between the current document and the URL being referenced.