Final answer:
An element is a pure substance composed of only one type of atom, making it the simplest form of matter, which cannot be broken down into anything simpler. The correct answer is D. element.
Step-by-step explanation:
The term that describes a pure substance made up of only one type of atom is element. As defined by chemistry, an element is a substance that contains only one kind of atom and cannot be broken down into a simpler type of matter. Elements are the simplest substances and they each have unique properties.
There are about 118 known elements, each identified by its atomic number and symbol on the Periodic Table of the Elements.
In contrast, a compound is a substance composed of two or more different types of atoms bonded together. Therefore, the correct answer to the student's question is D. element.
An element is a pure substance that is made up of only one type of atom. It cannot be broken down into other types of substances. Each element is composed of just one type of atom. An example of an element is oxygen or hydrogen.