Final answer:
Without additional context, it's unclear which Marty is referenced. If the question is about Marty from 'Back to the Future,' then his parents have been married for 20 years in the original timeline. Otherwise, the correct answer cannot be determined.
Step-by-step explanation:
To answer the question of how long Marty's parents have been married, more context is needed to identify this Marty as the question could relate either to a historical figure, a character in literature, or a cinematic character. For example, if we refer to Marty from the film Back to the Future, then the correct answer would be C) 20 years — assuming the context is the original timeline presented in the movie. Without specific context, the correct answer cannot be provided.