Integration testing is a level of software testing where individual units are combined and tested as a group. The purpose of this level of testing is to expose faults in the interaction between integrated units. Option D, 'It validates interactions between subsystems of your application,' accurately describes integration testing. Option A refers to system testing, which validates the complete and fully integrated software product. Option B describes unit testing, which focuses on the smallest parts of the software. Option C is not a defining characteristic of integration testing, as it can be performed manually or automated.