Answer:
Atom
Step-by-step explanation:
The atom is the simplest unit of matter that has all the properties of an element. It is made up of 3 subatomic particles: protons, electrons and neutrons.
Knocking out the other choices, the cell is the basic unit of life. An electron is only part of an atom, which has its own properties, while a compound is a combination of two or more elements that are chemically bonded.