The project team is highly experienced in module testing$-$but is uncertain about how to approach integration testing. When resourcing and accelerating projects using Agile methods, which statement BEST describes "integration test"?

a$.$ Ensuring small function or classes work properly

b$.$ Ensuring new functionality works immediately in existing system

c$.$ Verifying for customer entire system works correctly

d$.$ The practice of immediately ensuring each bit works in the system