Which symptom is commonly associated with motion sickness?

Nausea is a hallmark symptom of motion sickness, which occurs when there is a mismatch between the sensory input from the vestibular system (which senses balance and motion) and what the eyes perceive. This conflict creates confusion in the brain, leading to physical symptoms, with nausea being the most prominent. It often arises during activities such as traveling by car, boat, or airplane where motion is involved.

The other options do not align with the typical experiences of someone suffering from motion sickness. Increased alertness and enhanced concentration are generally not associated with the discomfort and distraction caused by motion sickness. Moreover, improved balance also contrasts with the disorientation that often accompanies motion sickness, which can lead to a sense of instability rather than balance. Thus, nausea is the most recognized symptom in relation to this condition.