Answer: Weight Watchers (you can eat anything, but count total calories)
Explanation: The other three options has the characteristics of a "fad" diet.
The grapefruit diet relies on a "magic" or "miracle" food.
The atkins diet and diets that limit what foods you can have and when are also fads because they are often used as a "quick fix" and are usually unrealistic lifestyle changes.
Weight Watchers (calorie counting) does not eliminate what foods you can have, but focuses on calorie input vs. output. It is a scientifically proven healthy weight loss method.