Final answer:
Annabeth's mother gave her an invisibility cap on her 12th birthday, resembling how mythical figures in Greek mythology often received magical items from the gods.
Step-by-step explanation:
The magic gift that Annabeth's mother gave her on her 12th birthday was d) An invisibility cap. This gift is drawn from Greek mythology where gods and heroes often receive magical items.
In the mythological tales, Athena, the goddess of wisdom and warfare, would be known to bestow gifts of magical items such as a reflective shield to Perseus, assisting him in his quest to defeat Medusa. In a similar vein, Annabeth, a character likely from a contemporary story inspired by such mythology, is gifted an item that confers an advantage, aligning with the narrative theme of divine aid.