Concept:The correct compound noun for the condition of feeling ill while travelling is "travel sickness".
Explanation:The blank needs a standard uncountable noun that names the nausea caused by motion in a car, plane, or boat.
"Travel sickness" is the conventional and widely accepted term in English.
"Travelling sickness" is not the standard compound form.
"Travel sicknesses" is incorrect because the condition is uncountable and does not take a plural.
"Traveller sickness" is not a recognised expression.
Therefore, the best choice is option A.
Answer:A. travel sickness