If an unknown substance cannot be broken down into simpler substances, it is
a. a compound.
b. an element.
c. made of one kind of atom.
d. both b and c

The correct answer is b. an element

The explanation:

when the ELEMENT is define as substance that cannot be decomposed (broken down) into simpler substances by ordinary chemical means. Examples: gold, iron, carbon, hydrogen, oxygen, nitrogen

Ex: diamond will be broken into carbon as well.

and SUBSTANCE: Material composed of only one kind of matter throughout (same atoms or same molecules).