Sarah Fernandezlopez says:

From Kert

Both are valid ways to do a sit up. There are also many more!
Essentially it just comes down to putting the hips and legs in different ranges of extension/flexion and rotation. And targeting strengthening the midline with the varying hip and leg position. Position of your legs will help determine what you are targeting.
Personally I don’t use a bent knee as I feel it places a lot of body weight in the top half, so it can make the sit ups way harder, and result in you falling back, rather than controlling the sit up back. Too much knee bend will also mean a reduced range of motion.
Bent knee versions often require a partner hold the feet, too.