A substance that is made up of all identical atoms is a chemical, specially a chemical compound.
Here are the reasons why the other choices cannot be the answer:
- An element would be a single atom and not a substance that consisted of multiple atoms.
- A compound would be any combination of 2 or more different elements that is held together by chemical bonds, meaning it isn't made up of all identical atoms
- An alloy is a substance that consists of more than one element where at least one of them is a metal, meaning it does not have identical atoms.
Let me know if you need any clarifications, thanks!
~ Padoru