Answer
The answer is The Master. ʻAbdu'l-Bahá, the eldest son of Bahá'u'lláh, the founder of the Baháʼí Faith, is commonly referred to as "The Master" in Baháʼí texts and by Baháʼís. This title signifies His position and authority as the appointed successor and interpreter of Bahá'u'lláh's teachings and the one who expounded and elucidated the Baháʼí Faith.