Identify the segment that contains a grammatical error. If there is no error, select 'No error.'
And thereupon he sudden remembered that he was not sleeping in his wife's room, but in his study, and the smile vanished from his face.

Correct option is A

Option (a) contains an error.
Error identification: "Sudden" should be "Suddenly."
Grammatical Rule used: "Suddenly" is an adverb and is used to modify the verb "remembered."
Corrected sentence: And thereupon he suddenly remembered that he was not sleeping in his wife's room, but in his study.
Meanings of all options:
Sudden: Happening or coming unexpectedly.
Suddenly: Quickly and unexpectedly (correct form).
Remembered: Recalled from memory.
Study: A room for reading or work.