Smallest particle in a chemical element or compound that has the chemical properties of that element or compound is
(A) Atom
(B) Molecule
(C) Mixture
(D) Element

Hint:Here we have to examine each of the options one by one to find the correct answer. Protons and neutrons are heavier and live at the core of the atom in the nucleus than electrons. In a cloud orbiting the nucleus electrons are very much light weight.

Complete step by step answer:
Atom: Atoms consist of a nucleus orbited by electrons consisting of protons and neutrons. Atoms consist of three elements: protons, neutrons and electrons, which are made up of even much smaller particles.
Molecule:A molecule is classified as two or more atoms of the same element that are bound together by a different element. A molecule may be homonuclear meaning that it consists of atoms in a single chemical element as with oxygen, or it may be heteronuclear, as with water, a chemical compound consisting of more than one element.
Mixture: A mixture is a product produced in such a manner that no chemical reaction happens when mixing two or more separate products. It is normally possible to break a mixture back into its original components. Majority of naturally occurring products are mixtures. No chemical change occurs during mixture of materials.
Element: A material which by chemical means cannot be broken down into simpler substances is known as an element. An element is made up of atoms of the same atomic number, i.e. each atom has in its nucleus the same number of protons as all the other atoms of that element. It is one of the simplest substances known.
So, from the above discussion we can conclude that the smallest particle in a chemical element or compound that has the chemical properties of that element or compound is a molecule.
Hence, option B is correct.

Note:There may be confusion between atoms and molecules, since atoms are even smaller than molecules and the question has asked us about the smallest substance. But here the question has also asked about chemical properties which only the molecule exhibits.