The correct answer is A. It is better to have loved and lost than never to have loved at all.
Explanation:
A theme refers to a statement that is usually concise and expresses one idea or belief held by the author of a literary work about any aspect of life such as love, imagination, solitude, death, etc. Additionally, in literary works, the theme is the main message the author wants the reader to understand. Examples of universal themes include "Nature is powerful" or "Love always prevails". considering this, the statement that is an example of theme is " It is better to have loved and lost than never to have loved at all" because this expresses an idea about one important aspect of life that can be used as the main message in a literary work. Also, other options expresse situation between people or characters but not themes.