The correct option is
C An unrecognisable enemy or danger
The given phrase means 'A treacherous person'.
Option A is incorrect. It does not have the required meaning, thus it is incorrect.
Option B is incorrect. 'A secret agent' does not mean 'a treacherous person'.
Option C is correct. 'An unrecognisable enemy or danger' means 'treacherous'. Thus, among the given options, it is the closest in meaning to the required one. Thus, it is correct.
Option D is incorrect. Unreliable does not mean treacherous. Thus, it is incorrect.