What is an ionic bond?
a. a bond between 2 atoms of similar electronegativities
b. a bond formed by sharing electrons
c. a bond formed from 2 atoms of the same element
d. a bond resulting from the transfer of 1 or more electrons

D = A bond resulting from the transfer of one or more electrons.

Explanation:

Ionic bond:

It is the bond which is formed by the transfer of electron from one atom to the atom of another element.  

Explanation:

Both bonded atoms have very large electronegativity difference. The atom with large electronegativity value accept the electron from other with smaller value of electronegativity.

For example:

Sodium chloride is ionic compound. The electronegativity of chlorine is 3.16 and for sodium is 0.93. There is large difference is present. That's why electron from sodium is transfer to the chlorine. Sodium becomes positive and chlorine becomes negative ion.  

while by sharing of electrons covalent bond is formed.

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.

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 negative.