Parallel testing is a type of testing where the tester reconciles the output of the new system to the output of the current system, in order to verify the new system operates correctly.
Two people testing an application on same machine.
Testing one functionality simultaneously on two different machine having same hardware configuration.
Testing two different functionality simultaneously.
Parallel testing compares the output of a new system against the current system’s output to verify correctness, which is exactly the definition provided. The other choices describe unrelated concepts, so the marked answer is correct.