Final answer:
Atoms in a molecule are held together by chemical bonds, and all molecules are composed of atoms. However, not all substances are composed of molecules, and while most substances are held together by various types of chemical bonds, there are exceptions.
Step-by-step explanation:
The statement that 'atoms in a molecule are held together by chemical bonds' (B) is true. Chemical bonds are indeed the force that holds atoms together within a molecule, allowing them to function as a single unit with specific chemical properties. Moreover, the statement that 'all molecules are composed of atoms' (C) is also true. By definition, a molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction.
However, the statement 'All substances are composed of molecules' (A) is not entirely true. While many substances, including various gases, liquids, and some solids, are molecular, not all are. For instance, ionic crystals such as table salt and metals do not consist of molecules but are arrays of ions or atoms held together by ionic or metallic bonds, respectively. Lastly, the statement 'All substances are held together by chemical bonds' (D) is broad but generally true, as chemical bonds are the forces that hold the atoms or ions within most substances together. This includes ionic, covalent, and metallic bonds, but there are exceptions, such as noble gases, which exist as separate atoms not connected by chemical bonds.