Answer : The correct option is, (D) an covalent bond between the atoms.
Explanation :
Ionic compound : These are the compounds where the atoms are bonded through ionic bond.
Ionic bonds are formed by the complete transfer of electrons. The atom which looses the electron is considered as electropositive atom and the atom which gains the electron is considered as electronegative atom.
The ionic bonds are formed between one metal and one non-metal atoms.
Covalent compound : These are the compounds where the atoms are bonded through covalent bond.
Covalent bonds are formed by the equal sharing of electrons. These bonds are commonly formed between two non-metal atoms.
In this given picture, the two electrons are equally shared between the two atoms. That means they form covalent bond between the two atoms.
Hence, the (D) an covalent bond between the atoms.