actually, i think it's none of the above.
first, too much caloric restriction causes problems. too little caloric restriction causes problems. finding a balanced, appropriate caloric intake is what works.
second, once the appropriate caloric is determined, then you have to find the appropriate percentages of protien, fat, and carbohydrates to suit your constitution and physical needs. The amounts of each can very, but these averages work well: 25-30% protien, 15-30% fats (saturated and unsaturated; no hydrogenated or transfats), and 40-60% carbohydrates (both simple and complex).
third, once you've determined the appropriate percentages of macronutrients needed in your diet, you have to find the appropriate sources for you. It is best to avoid all processed and 'junk' foods in favor of highly nutritious 'whole foods' from natural sources. One can be vegan or any form of vegetarian (or omni) and be healthier on whole foods than on any other diet.
finally, the other side of this equation is exercise or activity. some people have active jobs and do not need 'added' exercise. if they're eating the right amount of calories from healthy food sources, then they'll come into physical balance (appropriate weight for them). if a person over-exercises, this can lead to health problems. if a person underexercises--that is that their job is sedentary AND they don't do enough exercise--then this also leads to health problems. So, it is appropriate to find the right balance of activity as well as diet.