Atoms are the basic units of matter. The atom is the smallest unit of matter that can't be divided using any chemical means and the building block that has unique properties. In other words, an atom of each element is different from an atom of any other element.
The number of protons in an atom's nucleus defines what kind of atom it is. The number of protons in the nucleus is the atomic number and it defines to which chemical element the atom belongs.
Thus, the correct options are (A) and (D).
