You heat apiece of iron from 200 to 400 k. what
happens to the atoms' energyof random motion?

when a piece of iron is heated from 200 kelvin to 200 kelvin, the kinetic energy of molecule increases which causes the random motion of molecules in random direction faster than before.

Explanation:

when a piece of iron is heated from 200 kelvin to 200 kelvin, the kinetic energy of molecule increases which causes the random motion of molecules in random direction faster than before.  

we know that Temperature is inversely proportional to inter molecular attraction, thus whenever temperature increases the forces that bind the molecules together are dissipating hence particle moves in random motion.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

i think   the answer is c hope i'm right