If you are working out at home or don't have access to a bench, any sturdy object you can hold onto that will not move can work as a replacement.
Lower abdomen, from just above your belly button to your pubis. The lower half of the six pack abs.
Outer sides of your abdomen.
Muscles spanning the front of your thigh.
Upper abdomen, from just above your belly button to your sternum. The upper half of the six pack abs.
Bodyweight
This means that the exercise requires you to use your bodyweight as resistance but you also need other equipment to successfully perform the exercise.
Flat Bench Without Rack
If you can't find this bench, consider using a flat bench with rack instead. If your exercise requires you to use a barbell or bar, you cannot substitute. If you do not need to use either, you probably can substitute.
Exercises that target the same primary muscle groups and require the same equipment.
Exercises that target the same primary muscle groups with different equipment