Which of the following is a variant of the Harvard style of referencing?

The correct option is APA.

Referencing styles are generally categorized by how they attribute sources within the text. The Harvard style of referencing is the classic representative of the author-date system. In this system, in-text citations are placed within parentheses and consist of the author's surname followed by the year of publication (for example: Smith, 2020).

The APA (American Psychological Association) style is a specific, standardized referencing system that is built directly upon this author-date framework. Because it relies on the identical core method of using the author's last name and publication year for in-text parenthetical citations, it is considered a prominent variant of the broader Harvard referencing system, differing only in minor stylistic details such as punctuation, spacing, and capitalization rules.

To understand why the other options are not variants of the Harvard style, we can look at their formats:
- MLA (Modern Language Association) style uses an author-page format (for example: Smith 45) instead of the author-date system.
- IEEE (Institute of Electrical and Electronics Engineers) style is a citation-sequence system that uses numbered brackets (for example: [1]) to guide readers to a numbered reference list.
- Chicago style is most commonly associated with its notes-and-bibliography system which uses superscript numbers and footnotes, making it structurally distinct from the Harvard style.