Plants are watered on Wednesday. Hence the correct option is c.
The statement "If dinner is served on the same day as plants are watered" implies a correlation between serving dinner and watering plants. Since dinner is served on the same day, it narrows down the possibilities for both activities. Option C, "Plants are watered on Wednesday," is correct based on the given information.
This conclusion is drawn by understanding that the day dinner is served is the same day plants are watered, and Wednesday is the only day specified for plant watering. Other options can be ruled out as they do not align with the given condition or information. Hence the correct option is c.