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 \(CO_2\)
