Substances that cannot be broken down into simpler substances are called

Element

Explanation:

Element is a pure substance which is composed of atoms of similar elements.It can not be decomposed into simpler constituents using chemical reactions.Example: Gold

Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.It can be decomposed into simpler constituents using chemical reactions. Example: carbon dioxide

Thus as it given that substances cannot be broken down into simpler substances, thus they are called elements.

it should be all of them because it asks if you included all of those in your answer

explanation:

1. a)c-14 has two more protons

2. d)they have different numbers of protons, neutrons, and valence electrons; they are all gases at room temperature.

3. c) iron and nickel.