Question

what is the smallest particle of matter?
a cell
an atom
a proton
an element

Explanation:

A cell is the basic unit of life, not matter in general. An element is a pure substance made of one type of atom, not a particle. A proton is a subatomic particle, but atoms were historically defined as the smallest indivisible particle of matter (and for the scope of this basic question, it is the correct choice as protons are components of atoms, not the fundamental particle of matter in this context).

Answer:

B. an atom