Answer: Option (a) is the correct answer.
Step-by-step explanation:
A covalent bond is defined as the bond which is formed due to sharing of electrons.
For example, a carbon atom has 4 valence electrons and a hydrogen atom has 1 valence electron.
So, when a carbon atom chemically combines with a hydrogen atom then sharing of electrons take place between the two and it results in the formation of a methane molecule.
![C + 2H_(2) \rightarrow CH_(4)](https://img.qammunity.org/2020/formulas/chemistry/high-school/31t4iapz7g5xbd6kgq7npq850mqmf7ht7l.png)
Whereas a bond in which transfer of electrons take place from one atom to another is known as an ionic bond.
For example,
![2Na + Cl_(2) \rightarrow 2NaCl](https://img.qammunity.org/2020/formulas/chemistry/middle-school/k5qs96c7c5b0zt88gu06fnq5u5snb2ynfm.png)
Thus, we can conclude that in a covalent bond electrons that are shared between atoms holds atoms together.