Which CSS rule should be used to create three columns of equal width for the given box elements?

The chosen answer is based on the specific requirement to create three columns of equal width for the box elements. By using the CSS rule that applies to the #wrapper element with the setting display: grid; grid-template-columns: 1fr 1fr 1fr;, it takes advantage of the CSS Grid Layout.

In this context, 1fr stands for one fraction of the available space, so defining three columns with 1fr each essentially divides the total width into three equal parts. Each column will take up an equal share of the width of the grid container, making it ideal for achieving the desired layout of three equally sized columns.

The other options involve variations in the structure that do not meet the requirement for equal column widths. For instance, while some rules apply the grid display, they use different fraction values such as 2fr and 3fr, which would create unequal column widths, contrary to the request for equal distribution. Therefore, the correct choice is one that precisely aligns with the intent of creating three columns of the same width.