Rock Hard -

According to the ACE Fitness guide on core training , developing rock-hard abs requires targeting all four major abdominal muscle groups and incorporating back stabilizers.

To make your muscles visible, you must strip away the overlying body fat. This requires a slight caloric deficit while maintaining high protein intake to preserve muscle tissue. Rock Hard

To help you get exactly what you need, direct overviews for both interpretations are provided below. According to the ACE Fitness guide on core