Seven-month-old Del has achieved several motor milestones, including rolling over and sitting up without support. In addition to using his visual, skeletal, and muscular system, what else does Del use to help him accomplish these tasks?

The correct answer is - Vestibular organs in his inner ear.

Explanation:

Motor skills such as sitting up without assistance or support, rolling over, are need various body system to accomplish these tasks. Visual system for sense of position, skeletal system and muscular system for body movements.

Other than these three system there are one more essential part that help in performing such tasks that is Vestibular organs in his inner ear, which is helpful to maintain balance for performing and sensing the position in space.

Thus, the correct answer is -Vestibular organs in his inner ear.