How does an oxygen atom becomes oxide ion? 

By losing two electron. 

The correct answer is option 4. By gaining two electron.

Let's break it down:

1. Oxygen Atom (O): An oxygen atom typically has 8 electrons arranged in its electron cloud, with 2 electrons in its inner shell and 6 in its outer shell. Its electron configuration is \(1s^2 2s^2 2p^4\). The outer shell of an oxygen atom is not completely filled, making it reactive.

2. Gaining Electrons: In order to achieve a stable electron configuration, similar to that of a noble gas (like neon), an oxygen atom needs to gain electrons. Since the oxygen atom needs to gain two electrons to complete its outer shell, it's referred to as gaining a "pair" of electrons.

3. Formation of Oxide Ion \((O^{2-})\): When an oxygen atom gains two electrons, it now has a total of 10 electrons. The additional electrons fill up its outer shell, resulting in a stable electron configuration similar to that of a noble gas. This transformed oxygen atom is now an oxide ion \((O^{2-})\) because it has gained two negative charges (from the two extra electrons), giving it an overall charge of -2.

So, the process can be summarized as:
Oxygen Atom (O) + 2 Electrons → Oxide Ion \((O^{2-})\)

This transformation allows the oxygen atom to achieve a more stable configuration and contributes to the formation of various compounds, such as oxides.