I wanted to get an idea of which exercises are used for developing a six-pack stomach.
In this case, let's say that the individual already has a relatively small stomach with very little fat on it.
Which exercises could he use if he solely wanted to develop the stomach muscles into a "six pack"?
You may already know this, but it's my understanding that diet is a key factor in getting your six pack to reveal itself. In other words, you can have rock-hard abs, but nobody will see them if they are covered up in a layer of fat.
As has been pointed out, I didn't include information about exercises. Here is a link that should help educate people on how to strengthen their core, a worthy goal even for those not willing to commit to the diet needed to reveal a six pack (Wait, what NO beer?) As an aside, any claim of quick success -- as the title of this article suggests -- is absurd. The information, however, is valid. http://www.menshealth.co.uk/building-muscle/four-week-six-pack-plan
As @Adam Orth pointed out, a six pack is less a question of training as much as an excises in reducing body fat. For men, your body fat needs to drop to the region of < ~12%- 10% before the banding created by the rectus sheath cause the Rectus Abodminus to bulge between the tendinous intersections that traverse the muscle.
That said, any exercise such as press-up/plank, reverse crunch, ab-wheel, leg raises, will train and strengthen the muscle, on the proviso that the lumbar spline is straight (reverse crunch), which disengages leg/hip muscles and put the load onto the RA. Try this explainer https://www.youtube.com/watch?v=G9Q3XdeGTvM
Unfortunately, while you say you have a small stomach, if you cannot see your abs, you are not lean. Leaning out is up to your diet, not exercise.
The simplest thing to do if you want to lose fat and lean out quickly (this is assuming your entirely healthy and have nothing like diabetes or anything else ) stop eating all the foods with sugar in them.
Check the labels, nearly everything you buy packaged has sugar in it. If you can manage to cutout sugar from your diet, you'll lean out in a couple of months naturally.
Reducing carb intake (you need to up protein and fat to compensate for loss of calories) will accelerate this process. Again, assuming your healthy, your body doens't actually need carbs or sugar. It's perfectly capable of making glucose from fat.
If you really want exercises that are good for ab strength, consider squats and deadlifts. Contrary to what our culture would believe, your abs are for keeping intra-abdominal pressure at a level that compensates for the stress placed on our spine when we lift and move. In other words, they force your guts against your spine so you don't snap when you lift things.
They do not make you bend over, though they do assist with that motion, and are involved in helping the pelvice move among other torso motions. However, their primary function is to keep your guts in place, and support your back.
Thus lifting exercises that force you to tighten your whole body to complete the lift will greatly strengthen your core.
