Final answer:
Keats compares Psyche's beauty to the stars and Venus, using this to elevate her beauty above celestial objects, thus B) highlighting her unparalleled beauty.
Step-by-step explanation:
John Keats, in his reference to Psyche as "Fairer than Phoebe's sapphire-region'd star, / Or Vesper, amorous glow-worm of the sky;", is using vivid and illustrative language to B)highlight Psyche's unparalleled beauty.
Here, Keats compares Psyche to celestial objects, suggesting that she is even more beautiful than the stars and the evening star, Venus (referred to as Vesper).
This kind of comparison is a common poetic device used to elevate the subject's beauty by contrasting it with well-known symbols of loveliness and brilliance in nature.