0. When a ship crosses the International Date Line from west to East
Crossing the International Date Line from west to east causes the ship to go back a calendar day.
A. It loses one day – Correct. The date shifts back by one full day.
B. It gains one day – Incorrect. Gaining a day happens when traveling east to west.
C. It loses half-a-day – Incorrect. The shift is always by one full day, not a fraction.
D. It gains half-a-day – Incorrect. This option is not applicable in timekeeping.