D, because all others could affect other parts of your health. Don't crash diets, after so long your body can go into starvation mode. Exercising alone--- if there is not another person with you, you may be tempted to over-exercise. And C is incorrect because those pose big risks to your health, depending upon the combination or even duration/side-effects.
And D is correct because you're still eating--- just maybe more healthily and exercising, but not too much or too little.