A double covalent bond.
As their name suggest, ionic bonds are forces between ions of opposite charges. Ionic bonds involve the transfer of electrons from one atom to another. Such transfer can't occur between two carbon atoms. They belong to the same element and have the same tendency to gain or lose electrons. As a result, the bond between them cannot be ionic.
Ion dipole is a force between an ion and a molecule. The bond between the two carbon atoms is located within an organic molecule. The force between the two carbon atoms can't be ion dipole, either.
There are four valence electrons in each carbon atom. Each of them needs four more electrons to achieve an octet. They would achieve that octet by sharing four electrons with other atoms. Each shared electron pair acts as a covalent bond. Each hydrogen atom demands one more electron and would share only one electron with the carbon atom. The first carbon atom in this question shared two electrons with two hydrogen atoms. It needs to share two more with the other carbon atom so that it could achieve an octet. As a result, it would share two pairs of electrons, which will make a double covalent bond between the two carbon atoms.
