My answer is C I heard Ken wasn’t married, so that woman couldn'thave been his wife. Could with perfect infinitive is used for speculating or guessing about the past.That means that It couldn't be possible that that woman was his wife since he wasn't married.
C: When you saw Ken last week, he was kissing with a woman. And you thought it was his wife. But today, you heard Ken wasn't married, so that woman couldn't have been his wife. (Maybe it was his girlfriend)