#3 Bird Dog or Aeroplane
Think of this as an upside-down dead bug. Start in a tabletop position, with your shoulders over wrists and hips over knees. Engage your core while simultaneously lifting your right arm and left leg. Your foot should be flexed as you kick back, and your palm should face in towards your body. Pause for one second when your arm and leg are at the same height as your torso, and then bring your elbow and knee to touch underneath the body. Repeat on the other side for one rep, and do five reps for one set.
Again, this exercise not only works out your abs but it also works your glutes and your hamstring. It makes them tight and a lot stronger. The bird dog while starting out is an intense exercise but as your fitness levels increase you graduate to other exercises. Thus this is a great starting block. One should to 20 sets on each leg and about 3 repetitions of this exercise.