Which pair of elements will form a covalent bond?
a. hydrogen and chlorine
b. sodium and chlorine
c. copper and bromine
d. zinc and bromine

Option (A) is the correct answer.

Explanation:

A covalent bond is formed when there occurs sharing of electrons between atoms.

For example, in hydrogen atom there is one electron in its orbit and in a chlorine atom there is 7 valence electron. So, in order to attain stability both hydrogen and chlorine share electrons when they come close to each other.

Whereas except hydrogen and chlorine rest of the given atoms will form ionic bond, that is, bond formed by transfer of electrons.

Thus, we can conclude that a pair of hydrogen and chlorine will form a covalent bond.

well in order to answer this question you have to provide more information < 3

"among the transition metal elements, manganese and zinc have a slight lowering of electronegativities. the reason behind this is that the nuclear charges of these elements are slightly weaker than the other transition metal elements due to the way their valence electrons are filled up. both manganese and zinc have their valence electrons filled up to the 4th shell with 1 pair of electrons (4s2) completely occupying it. although the electrons in the 3rd shell (3d) are yet to be paired since the outer most shell is filled up, the ability of the nucleus to attract electrons is lessened. hence,   the electronegativity (or the ability to attract electrons) is lessened."