There's no such thing as the "lower" abddominal muscles. Your abdominus rectus muscle runs from your groin to your sternum and is one muscle. You can't work part of a muscle.
Generally speaking, whole-body moves like Plank and its variants are safer for lower back risk than crunching or curling.
If you're trying to reduce fat in the region, though, no exercises will target fat from any specific part of the body. Include whole-core exercise because it's good for you, and keep up the appropriate calorie deficit diet, and the fat will come from wherever your genetics determines it will.
Deb, in New Zealand