An atom: is the smallest whole unit of matter can form one or more elements is the smallest whole unit of matter with the properties of a specific element all of the above

The answer is ‘the simplest unit of an element that still retains the properties of that element’. An atom, however, is also made up of subatomic particles such as neutrons and protons (that make up the nucleus), and electrons  that make up the orbital cloud. An atom 100 picometers in diameter.