The ___ is the smallest unit of matter.
a) Element
b) Atom
c) Molecule
d) Electron

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Hint: In chemistry the entity which cannot be divided further into smaller constituting entities is called the smallest unit of matter. It should also retain all of the chemical properties of an element.

Complete step by step solution:
a) Element: An element is a substance that is made entirely from one type of atom and hence they can be divided into atoms. Thus, they are not the smallest unit of matter.
b) Atom: Atom is the building block of a matter. The basic particles that make up an atom are electrons, protons, and neutrons. Though it can be divided into smaller constituents, none of them retain the chemical properties of the matter. For example, an electron from a hydrogen atom will behave similar to an electron from an oxygen atom.
c) Molecule: Molecules are a group of atoms bonded together; representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction this implies that they can be further divided into atoms.
d) Electron: It is the sub atomic particle of an atom but it cannot retain its chemical properties.

From the above information we can conclude that atom is the smallest unit of matter and hence option B is correct.

Note: An atom is the smallest constituent unit of the ordinary matter that constitutes a chemical element. Every solid, liquid, gas, and plasma we encounter is composed of neutral or ionized atoms. These are extremely small, typically having a radius of 100 picometers.