To insert a comment in a CSS (Cascading Style Sheets) file, you can use the following syntax:

A. /* this is a comment */

This option is correct. In CSS, you can insert comments using the /* / syntax. Anything between the opening / and closing */ will be treated as a comment and will not affect the actual styling of the elements on the page.

B. ' this is a comment

This option is incorrect. In CSS, using single quotes (' ') is not the correct syntax for comments. It may cause an error if used in this way.

C. // this is a comment //

This option is incorrect. In CSS, the double forward slash (//) is not the correct syntax for comments. It is used in some programming languages like JavaScript, but not in CSS.

D. // this is a comment

This option is incorrect. As mentioned earlier, the double forward slash (//) is not the correct syntax for comments in CSS.

So, the correct answer is A. /* this is a comment */

Using this syntax, you can insert comments in your CSS file to provide explanations or notes for yourself or other developers working on the code.