Which can form a double covalent bond?
cl
h
n
ca
c
o

Oxygen

Explanation:

First of all, let's remember that covalent bonds are formed between nonmetal atoms. Given the list of the atoms, calcium is the only metal (it belongs to group 2A, alkaline earth metals). This means calcium doesn't form covalent bonds, it forms ionic bonds. The remaining atoms are all nonmetals and form covalent bonds.

To avoid ambiguity and several choices possible for this problem, we'll assume that we're only talking about diatomic molecules (molecules consisting only of two atoms) in this question.

In covalent bonding, atoms join together to have their valence shells filled, that is, to have octets in their shells (except hydrogen which can only have up to 2 electrons in its shell). Let's look at how many bonds each of these nonmetals will form in a diatomic molecule:

Chlorine is in group 7A, it has 7 valence electrons, meaning it only needs 1 electron from a covalent bond to have an octet. This implies chlorine will have a single bond.Hydrogen is in group 1A, it has 1 valence electron and the maximum it can have in its valence shell is 2 electrons, so it also lacks 1 electron that will be provided by the other atom in a single bond.Nitrogen is in group 5A (it has 5 valence electrons). This means it needs 3 more to gain an octet, those would come from a triple bond.Carbon is in group 4A (it has 4 valence electrons). It needs 4 more electrons to gain an octet. A quadruple bond would not be possible, the greatest carbon can make is a triple bond in CO having a lone pair remaining on carbon.Oxygen is in group 6A (it has 6 valence electrons). It needs 2 more electrons to gain an octet. This implies it would make a double bond to obtain two more electrons from the other atom and acquire an octet.