The correct answer is his loyal gratefulness.
Indeed, as Crusoe is stranded in the island, Carib cannibals arrive with some prisoners that they kill and eat. One of their Indian captives manages to escape and flees and Crusoe saves him by ambushing and killing his two pursuers. The escapee is so grateful with Crusoe that he becomes his grateful, loyal servant and is named Friday by Crusoe, for the day when the his life was saved by the latter.
This is of course a very royalist analogy of the British Monarchy, where Crusoe represents the British benevolent King, who protects the loyal illiterate subjects (represented by Friday) from the rabble or the seditious (represented by the cannibals).