The correct answer is "They recognize that she loves a human man".
Ariel and the others slyphs leave Belinda behind once they find out that she has feelings for a human male. Ariel is able to see Belinda's desire to be with this man by penetrating deep into her mind. They are afraid that Belinda is going to lose her powers, since the slyphs will only protect her for as long as she rejects her involvement with the human race, which she was no longer able to do because of her romantic feelings for a man.
Hope this is helpful to you!