In UML Sequence Diagrams, a "neg" (negative) combined fragment is used to specify invalid or forbidden interactions. Therefore:
* A.Incorrect, because a "neg" fragment defines an invalid interaction, so following it by normal positive constructs does not affect its interpretation of invalidity.
* B.Incorrect, as nesting a "neg" fragment within another "neg" fragment does not inherently result in valid traces.
* C.Correct. Normal positive constructs followed by a "neg" fragment define some sequences of interactions as invalid, which is the intended use of the "neg" fragment.
* D.Incorrect, because an "assert" fragment is used to specify required interactions, and containing a "neg" fragment within it does not make the invalid traces valid.
References:
* UML Specification: Interactions section, particularly the explanation of combined fragments and their meanings.
* The negative combined fragment and its semantics are discussed in the UML 2.5 Documentation, under the Sequence Diagrams and Combined Fragments sections.