The basis of an covalent bond is
a. the sharing of an electron pair
b. electrical attraction between oppositely charged ions.
c. absorption of energy
d. absorption of water into their solid structures

A) the sharing of an electron pair

Explanation:

Covalent bond:

It is formed by the sharing of electron pair between bonded atoms.  

The atom with larger electronegativity attract the electron pair more towards it self and becomes partial negative while the other atom becomes partial positive.

These compounds are insoluble , hard, having low melting and boiling points and have low conductivity.

For example:

In water the electronegativity of oxygen is 3.44 and hydrogen is 2.2. That's why electron pair attracted more towards oxygen, thus oxygen becomes partial negative and hydrogen becomes partial positive.