The correct option is C mobile
Sedentary means (of work or a way of life) characterized by much sitting and little physical exercise. The opposite of sedentary is mobile, or able to move or be moved freely or easily. Hence, Option C is correct. Among the rest of the options, laborious means requiring considerable time and effort, stationary means not moving or not intended to be moved, efficient means (of a person) working in a well-organized and competent way. These options do not express the intended antonym, hence incorrect.