The sit-up is the most basic trunk and hip flexion ab exercise.
Lie supine on the floor with the knees and hips bent to place the feet flat on the floor (the feet can be anchored under a weight or rack). Hold the fingers at the sides of the head. Crunch the trunk and then lift it to bring it upright. Return to lying flat while maintaining tension on the abs and repeat.
The sit-up is a basic but effective trunk and hip flexion exercise for trunk stability.
Sit-ups should be done for sets of 10-30+ reps; weighted sit-ups can be done for sets of 8-15 reps.
The sit-up can be done with or without the feet anchored, with additional weight (ideally held behind the head/neck), with slow tempos, on a decline bench
and with pauses in a partially-sitting position.