What aquatic exercise involves floating on the back with knees resting on the pool deck?

The aquatic exercise being described is characterized by the movement of floating on the back while positioning the knees on the pool deck. This position allows for support and stability, enabling participants to engage their core and work on alignment in a gentle manner.

The curl-up exercise, typically performed in water, focuses on activating the abdominal muscles to lift the upper body while the lower body remains supported. This position can help reduce strain on the spine compared to traditional land-based exercises, making it suitable for a variety of fitness levels.

The other options do not fit the described activity. Flutter kicks involve an alternating leg motion in a vertical position, water jogging focuses on moving through the water in a running motion, and sculling refers to a technique of moving the arms in a circular motion while floating or swimming—none of which include the back floating position with knees on the deck. Therefore, curl-ups are indeed the exercise that matches the given description perfectly.