Which of the following best exemplifies 'mendacity'?

The term 'mendacity' refers to the quality or state of being untruthful; it embodies a tendency to lie or deceive. Therefore, a habit of lying is a direct manifestation of mendacity, making it the most fitting choice among the options provided.

In contrast, the other options all highlight positive attributes associated with honesty and truthfulness. Truthfulness in reports, integrity in promises, and sincerity in relationships all emphasize commitment to conveying the truth or maintaining honesty, which is the opposite of mendacity. These notions are centered around trustworthiness and reliability, unlike mendacity, which denotes a lack of these qualities through a consistent pattern of deceit. This distinction firmly positions 'a habit of lying' as the best example of mendacity.