What signifies a sailor's official transition to become a "Shellback"?

Becoming a "Shellback" is a significant milestone in a sailor's naval career, marked by participation in a specific ritual known as the "Line Crossing Ceremony." This ceremony traditionally takes place when a ship crosses the equator for the first time. During this event, sailors who have never crossed the equator before are initiated into the ranks of Shellbacks, symbolizing their transition from "pollywogs" (those who have not crossed) to Shellbacks (those who have). The ceremony often includes a variety of activities that are both playful and challenging, reinforcing camaraderie and a sense of achievement among sailors.

The other choices do not accurately represent the process of becoming a Shellback. For example, passing a test, while it might be important in a sailor's training and qualifications, is not tied directly to the ceremonial transition into this specific group. Graduating from boot camp is a critical step in a sailor's journey but does not involve the unique traditions associated with crossing the equator. Similarly, receiving a merit badge pertains more to specific skills or accomplishments rather than the ceremonial transition into a Shellback. Thus, participation in the ritual is the defining action that signifies this significant naval milestone.