Explanation:

The correct answer is option 'D' which is COLS.

Frameset:
- A frameset is an HTML element that allows for the division of a webpage into multiple rectangular sections called frames.
- Each frame contains a separate HTML document and can display different content simultaneously.

The COLS attribute:
- The COLS attribute is used to define the number of columns in a frameset.
- It is used within the tag to specify the layout of the frames.
- The value of the COLS attribute represents the width of each column, defined in pixels or as a percentage of the viewport.

Example:
Let's consider an example to understand how the COLS attribute works:

```html





```

- In this example, the COLS attribute is set to "25%, 50%, 25%". It means that the frameset will be divided into three columns, with the first and third columns occupying 25% of the total width, and the second column occupying 50% of the total width.
- The tags inside the frameset specify the source files for each frame.

Points to remember:
- The COLS attribute is used in conjunction with the tag.
- It is used to define the layout of frames in terms of columns.
- The value of the COLS attribute represents the width of each column.
- The total sum of column widths should usually equal 100% or the width of the viewport.

Summary:
The COLS attribute is used in a frameset to define the number and width of columns. It allows for the division of a webpage into multiple frames, each displaying different content simultaneously.