What does Negative Testing verify in an application?

Negative testing is a critical aspect of software testing that focuses on assessing how an application behaves under abnormal or unexpected conditions. This form of testing specifically evaluates the application's response to invalid input, unexpected user behavior, or situations that it was not designed to handle. By doing so, negative testing helps identify vulnerabilities and ensures that the application can gracefully handle errors, rather than crashing or producing misleading results.

Focusing on invalid inputs allows testers to verify that the application implements proper error handling, logging, and user feedback mechanisms. This is essential for maintaining the application's integrity and providing a positive user experience even when things go wrong. For instance, if a user inputs a string into a field that requires a numeric value, negative testing checks whether the application correctly rejects the input and provides a helpful error message.

The options concerning normal conditions, valid inputs, and efficiency do not encapsulate the essence of negative testing. They instead relate to other forms of testing, which are more focused on functionality, performance, and user behavior under standard circumstances. Thus, C is the most appropriate choice, as it directly addresses the core objective of negative testing in evaluating the application’s robustness against invalid or unexpected inputs.