Nope. you can't lose fat in a particular place. If you burn more calories than you consume, then you will lose weight but it is your body that "decides" where to remove the fat from. Situps will help you tone the muscles in that area, but thats it.
Sit ups will help build up the muscle around your midsection but it will not burn fat. Aerobic exercise is the only way to burn fat. The best way to fight the tummy bulge is to combine aerobic exercise with sit ups.
When you work out individual muscle areas like squats, situps, and pushups, you 'tone' that area. obviously you burn energy simply being active, so technically speaking yes you can flatten your stomach by doing situps.
Sit-ups do use your ab muscles, but they do not burn a lot of fat. If you did a lot of sit-ups you would increase the size of your abs, maybe even get a 'six pack'. The problem is, however, you would still have all that fat over them! A good cardio routine will get rid of fat.
It's not true at all. Of course you can lose weight in one particular place, though you will simultaneously lose weight in other places. The problem here is more one of semantics than one of what you will actually achieve if you do sit-ups. Do the sit-ups; you will lose weight everywhere, including one particular place.