When two atoms are held together because their valence electrons orbit around both of them (are shared), they are united by?

When two atoms are held together because their valence electrons orbit around both of them (are shared), they are united by covalent bond.  

Covalent bond is a chemical bond that is formed between two atoms by equal sharing of electrons. Electron pairs that are shared between different atoms are called bonding pairs.In covalent bond formation, valence electrons orbit around the atoms that are shared between them.

i'm pretty sure it is letter c.) car

c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.