Final answer:
Achilles' mother was Thetis, who protected him by dipping him in the River Styx, making him nearly invulnerable except for his heel, and by hiding him disguised as a girl on the island of Skyros.
Step-by-step explanation:
The correct answer to the student's question is B) Achilles' mother was Thetis. She tried two different methods to protect her son. The first method was dipping him into the River Styx, which made him nearly invincible, except for the heel by which she held him. The second was by disguising him as a girl and hiding him among a princess's court on the island of Skyros. These acts were to shield Achilles from a prophecy foretelling his death should he participate in the Trojan War.