Which type of testing involves validating the system's functionality against the requirements?

Functional testing is the process of verifying that a software application behaves as expected according to the specified requirements. This type of testing focuses on assessing whether the system's features and functionalities work correctly and meet the defined specifications. It involves testing individual functions of the software by feeding them input and examining the output against the expected results as described within the requirements documentation.

The main goal of functional testing is to ensure that all functions of the software operate in conformance with the requirements. This can include unit testing, integration testing, system testing, and acceptance testing, among others, all geared towards validating the functionality of the system.

In contrast, non-functional testing evaluates aspects such as performance, usability, reliability, and other criteria not directly related to specific behaviors or functionalities of the system. Integration testing focuses on examining how different components or systems interact with each other. System testing involves testing the complete integrated system to ensure it meets its specified requirements, but it encompasses both functional and non-functional aspects. However, functional testing is the most direct answer in this context, as it specifically validates the functionality against requirements.