What is the best exerice for your lower belly.?

Lower abdominals. It depends on your goal. If you want to strengthen your lower abs, core excersizes like squats, planks, and hanging knee raises will help. If you are interested in losing fat on that area, targeted excersizes do not work. Aerobic exersize, and eating a healthy diet should work on most people. Be sure to consult your doctor before starting a new exersize program.
No targetting. It is better to exercise the whole body, preferably aerobic exercises than trying to exercise one area. Attempting to target one area will not selectively cause fat loss in that area.