What is an example of an idiosyncrasy?

An idiosyncrasy refers to a distinctive or peculiar feature or characteristic of an individual, often relating to habits or behaviors that are unique to them. This term encapsulates the idea that each person may have their quirks or particular ways of doing things that set them apart from others.

In the context of the provided options, a unique habit stands out as the correct choice because it directly relates to the personal nature of an idiosyncrasy. Unlike common traits, general characteristics, or universal behaviors that might apply to a wider group, a unique habit emphasizes the individuality and specificity associated with idiosyncrasies. This highlights how personal quirks can illustrate a deeper understanding of someone's character or personality.