Explanation: Regression testing can be performed any time when a program needs to be modified either to add a feature or to fix an error. It is a process of repeating Unit testing and Integration testing whenever existing tests need to be performed again along with the new tests. Regression testing is performed to ensure that no existing errors reappear, and no new errors are introduced. Answer: D is incorrect. The acceptance testing is performed on the application before its implementation into the production environment. It is done either by a client or an application specialist to ensure that the software meets the requirement for which it was made. Answer: A is incorrect. Integration testing is a logical extension of unit testing. It is performed to identify the problems that occur when two or more units are combined into a component. During integration testing, a developer combines two units that have already been tested into a component, and tests the interface between the two units. Although integration testing can be performed in various ways, the following three approaches are generally used: The top-down approach The bottom-up approach The umbrella approach Answer: C is incorrect. Unit testing is a type of testing in which each independent unit of an application is tested separately. During unit testing, a developer takes the smallest unit of an application, isolates it from the rest of the application code, and tests it to determine whether it works as expected. Unit testing is performed before integrating these independent units into modules. The most common approach to unit testing requires drivers and stubs to be written. Drivers and stubs are programs. A driver simulates a calling unit, and a stub simulates a called unit.