Class BioDiesel in turn depends on the class Viscosity() for it to be initialized. So you have to know if statement new Viscosity() is legal, or may be the class Viscosity uses a getInstance method to pass a new instance to you, or may be the class Viscosity uses a getViscosityInstance to pass a new instance, or may be... well you get the picture.
If your class is a user class then yes the coupling is tighter. The real exam will not ask you - what if String is a module outside the java.lang package. In fact the coupling question that i encountered on the exam was programmatic and a drag and drop. Yikes ! However they are easy to answer. Like the Kathy and Bert book mentions - the question will be direct and simple.