What is the most appropriate form of exercise for abdominal muscle?

Abs: I don't think that there is 1 best type of exercise to work on the abdomen. Also what is your goal w your abs? To tighten your abdominal muscles consider doing crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on your back. Insure that you are engaging in regular aerobic exercise. A trainer at a gym can show you lots more.