The correct answer is A. food.
Exercise is to gym as eating is to food. Gym is a place where people go to exercise, and food is what people eat. Both exercise and eating are necessary for good health.
Option B, dieting, is incorrect because dieting is a specific type of eating that is intended to lose weight. Gym is not a place where people go to diet.
Option C, fitness, is incorrect because fitness is a general state of health and well-being. Gym is a place where people go to exercise, which can help them to improve their fitness.
Option D, restaurant, is incorrect because a restaurant is a place where people go to eat. Gym is not a place where people go to eat.