What testing method injects invalid or unexpected inputs into an application to observe its response?

The correct choice is the method known as a Fuzzer. A Fuzzer is a testing technique specifically designed to input a wide range of invalid, unexpected, or random data into an application to uncover vulnerabilities, errors, or unexpected behaviors. The main goal of using a Fuzzer is to test how the application handles such inputs, which can help identify security weaknesses and stability issues that might not be apparent through traditional testing methods.

This technique is essential in security testing, as it can reveal how the application reacts to malicious inputs that a potential attacker might use in real-world scenarios. By automating the input of a variety of flawed data, fuzzers help developers and security professionals discover issues like buffer overflows, crashes, and other security vulnerabilities that could be exploited by attackers.

Other methods mentioned serve different purposes. Validation Testing focuses on ensuring that an application meets specified requirements and functions correctly under normal conditions. Penetration Testing simulates real-world attacks to evaluate the security of the application and its defenses rather than specifically using unexpected inputs. Integration Testing assesses how different modules of a software application work together, rather than how they react to unexpected input.