Answer: The correct answer is bonds.
Step-by-step explanation:
For the given options:
Option 1: Ions
Ions are defined as the chemical species which are formed when an atom looses or gains electrons.
When an atom looses its electron, it forms a positive ion known as cation and when an atom gains electron, it forms a negative ion known as anion.
Option 2: Bonds
Bonds are formed when two atoms combine by the attraction of atoms or by the sharing of electrons between the atoms.
When bond is formed by the attraction of atoms, it forms ionic bond and when sharing of electrons takes place between the atoms, it forms covalent bond.
Option 3: Isotopes
Isotopes are defined as the chemical species which have same atomic number but differ in mass number.
Option 4: Shells
Shells are defined as the orbits around the nucleus where electrons are present.
Hence, the correct answer is bonds.