Question

____ are made up of one kind of atom which can not be broken into simpler substances.
(A) Compound
(B) Molecule
(C) Ion
(D) Element

Answer 1

An atom consists of a nucleus having protons and neutrons. Electrons in an atom are orbiting the nucleus. Molecules are made by combining atoms. Elements are all the substances mentioned in the periodic table. Compounds are made by combining two or more elements. Ions are the charged particles made from elements.

Complete Step by step solution
Chemical compound is a chemical substance composed of many identical molecules. Chemical compounds are of four types depending upon how constituent particles are held together. They are molecules held by covalent bonds, compounds held by ionic bonds, intermetallic compounds headed by metallic bonds, and certain compounds also have a co-ordinate bond. In a compound, different types of atoms are present in a specific stoichiometric proportion.
A molecule is an electrically neutral group containing two or more atoms. They differ from ion as ion contains charge and molecules do not contain. Molecules can be homonuclear or heteronuclear. In a homonuclear molecule, a molecule consists of one chemical element and in a heteronuclear molecule, a molecule consists of more than one chemical element. Molecules are the smallest unit of the compound.
Ion has a net electrical charge and it can be a particle, atom or molecule. Ion gets a net electrical charge because the total number of electrons is not equal to the total number of protons. A cation is a positively charged ion and an anion is a negatively charged ion. There are monatomic ions and polyatomic ions. In monatomic ions, there is only a single atom and in polyatomic ions there are more than one atoms present.
Element is a pure substance and it cannot be broken down by chemical means. Elements have atoms containing equal numbers of protons. Of the total 118 elements identified, 94 are naturally occurring elements and remaining 24 elements are synthetic elements. Hence, elements are made up of one kind of atom which cannot be broken into simpler substances.
Therefore, the correct answer is option D.

Note
Neutral or ionized atoms are present in every solid, liquid, gas or plasma. Elements, molecules and compounds are classified as neutral pure substances while ions are electrically charged pure atoms present in molten/liquid state.