Crunch
Crunch is a classic core exercise that explicitly trains your abdominal muscles - part of your core. It also includes your oblique muscles on the sides of your trunk and the muscles in your pelvis, lower back, and hips.
General Situp is a classic abdominal exercise that strengthens and tones the core-stabilizing abdominal muscles. Situps work the rectus abdominis, transverse abdominis, obliques, and your hip flexors, chest, and neck.
Crunch is a classic core exercise that explicitly trains your abdominal muscles - part of your core. It also includes your oblique muscles on the sides of your trunk and the muscles in your pelvis, lower back, and hips.
General Plank is a classic exercise that strengthens your body from head to toe. In particular, the plank helps strengthen your core muscles, including your abdominals and lower back.