The standard and universally recognized syntax for a comment in a Cascading Style Sheets (CSS) file is to enclose the comment text between / and /. This syntax allows for both single-line and multi-line comments. Any text within these delimiters will be ignored by the browser's CSS parser, making it suitable for adding notes, explanations, or temporarily disabling code.

B. is not a valid comment syntax in CSS, HTML, or JavaScript. It resembles a DOCTYPE declaration but is syntactically incorrect.

C. is the correct syntax for creating a comment in an HTML document, not in a CSS file.

D. // this is a comment is a single-line comment syntax used in JavaScript and other programming languages. It is not part of the official CSS specification.

1. W3C. (2021, December 21). CSS Syntax Module Level 3. Section 3.3. Comments. Retrieved from https://www.w3.org/TR/css-syntax-3/#comments.

2. Mozilla Developer Network (MDN). (n.d.). CSS building blocks: CSS syntax. MDN Web Docs. Retrieved from https://developer.mozilla.org/en-US/docs/Learn/CSS/Buildingblocks/Syntax. (See the "Comments" section).

3. Massachusetts Institute of Technology (MIT) OpenCourseWare. (2014). 6.813/6.831 User Interface Design and Implementation, Lecture 4: CSS. Retrieved from https://ocw.mit.edu/courses/6-813-user-interface-design-and-implementation-spring-2011/resources/mit6813s11lec04/. (Slide 10 discusses CSS syntax including comments).