The correct answer is Ambiguity problem for CFGs.

Key Points

• The Ambiguity problem for CFGs (Context-Free Grammars) is undecidable.
  • A context-free grammar is ambiguous if there exists a string that can be generated by the grammar in more than one way (i.e., it has more than one distinct parse tree).
  • The ambiguity problem asks whether a given CFG is ambiguous.
  • This problem is undecidable, meaning there is no algorithm that can determine the ambiguity of any CFG in general.

Additional Information

• Other problems listed in the options are decidable:
  • The membership problem for CFGs is decidable. This problem asks whether a given string can be generated by a CFG.
  • The finiteness problem for FSAs (Finite State Automata) is decidable. This problem asks whether the language recognized by the FSA is finite.
  • The equivalence problem for FSAs is decidable. This problem asks whether two FSAs recognize the same language.