Explain what an exoskeleton is and what it is made of.

A rigid outer lair covering for the body in some invertebrate animals, especially arthropods, providing both support and protection. and is made of chitin, a substance produced by many non-arthropods as well.

Hard "thick structure" 
Example: crabs and insects mostly have them. 

There made of bones, provide armor and protection of that animal

Your answer is c. explanation: plasma  is the fourth state of matter, derived from the gaseous state of matter.  plasma  can contain free  electrons  and/or  ions. while  plasma is never neutral, it can either be positively or negatively charged.  gases, however, are electrically neutral.

Aform of energy that is reflected or emitted from objects in the form of electrical and magnetic waves that can travel through space. there are many forms of  electromagnetic  energy including gamma rays, x rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves.