The process of ‘Nitrification’ in the nitrogen cycle involves:

The correct option is Conversion of ammonia to nitrites and then nitrates.

Step-by-step Explanation:

1. Understanding the Nitrogen Cycle:
The nitrogen cycle is a biogeochemical process through which nitrogen moves through both biotic (living) and abiotic (non-living) environments. Nitrogen gas makes up about 78% of the Earth's atmosphere, but plants and animals cannot use atmospheric nitrogen gas ($N_2$) directly. It must undergo various biological and chemical transformations.

2. What is Nitrification?
Nitrification is a crucial biological process in the nitrogen cycle carried out primarily by specialized aerobic soil bacteria. It involves the two-step oxidation of ammonia (${\mathrm{NH}}_3$) or ammonium ions (${\mathrm{NH}}_4^{+}$) into nitrites and subsequently into nitrates.

3. The Two Steps of Nitrification:
• Step 1: Oxidation of Ammonia to Nitrite:
Ammonia is first converted into nitrite (${\mathrm{NO}}_2^{-}$) by nitrifying bacteria known as Ammonia-Oxidizing Bacteria (AOB), such as Nitrosomonas.
$2{\mathrm{NH}}_3+3O_2\to 2{\mathrm{NO}}_2^{-}+2H^{+}+2H_{2}O$

• Step 2: Oxidation of Nitrite to Nitrate:
Nitrite is then oxidized into nitrate (${\mathrm{NO}}_3^{-}$) by Nitrite-Oxidizing Bacteria (NOB), such as Nitrobacter.
$2{\mathrm{NO}}_2^{-}+O_2\to 2{\mathrm{NO}}_3^{-}$

4. Analyzing the Options:
• Conversion of nitrogen gas to ammonia: This process is called Nitrogen Fixation.
• Conversion of ammonia to nitrites and then nitrates: This is the exact definition of Nitrification.
• Release of nitrogen back into the atmosphere: This process is known as Denitrification.
• Absorption of nitrogen by animals: Animals gain nitrogen by consuming plants or other animals (Assimilation).

Hence, the process of 'Nitrification' involves the conversion of ammonia to nitrites and then nitrates.