Question

A substance that contains only one kind of matter is known as a(n)
A. atom.
B. element.
C. molecule.
D. compound.

Answer 1

Elements are composed of only one type of atom, so element is the term you need

Answer 2

Answer: B. element.

Explanation:

An atom is the smallest unit of any matter which may or may not have independent existence.

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: Copper $(Cu)$

Molecule is the smallest unit of any matter which always have independent existence.

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: water $CO_2$