Organ system is the answer
This diagram could represent a B.
protein because if you look at the bonds you will see an R group and R groups are typically found in proteins.
The center of an atom is a nucleas. you can find this answer in most textbooks on science
The epiglottis is a flap in the back of your tongue that prevents food from entering the larynx. When your mouth is closed, the epiglottis is in place and blocks the larynx opening. If your mouth is open, the flap lifts and the larynx is no longer blocked so you can breathe. This is why it's important to not chew with your mouth open... if you do, then food can go down the larynx by mistake.