A(n) is a pure substance that can't be broken down into simpler substaces by chemical or phyical means. a. chemical b. mineral c. element d. atom

A(n) element is a pure substance that can't be broken down into simpler substances by chemical or physical means. 

For example, if we were to speak about the 'chemical element' based on  which would  be oxygen, we can see and understand that this kind of substance can not be broken down into smaller pieces because this is an element, and nothing can stop this from getting smaller, and also shrinking in size and also having it be destroyed.

But that would just be an example, but therefore, your correct answer would be "element".