Sea sickness is caused by the motion of the ship, specifically the rocking and swaying movements. The correct answer is option 3, the stomach. When the ship moves, our body feels the instability and tries to adjust to it. This causes a disruption in the digestive system, leading to feelings of nausea and dizziness. The stomach plays a key role in this process as it is responsible for processing and breaking down food. The motion of the ship can cause the stomach to become unsettled, resulting in sea sickness. It is important to note that while the internal ear (option 1) also contributes to our sense of balance, in the case of sea sickness, the primary impact is on the stomach.