The identity of the faithful and discreet slave is discussed in The Watchtower, July 15, 2013, pages 21 - 22, paragraphs 8 - 10.
وَتُوضِحُ ٱلْمَقَالَةُ ٱلسَّابِقَةُ فِي هٰذِهِ ٱلْمَجَلَّةِ هُوِيَّةَ ٱلْعَذَارَى .‏