The answer is option 'A', acid-soluble pool.

Explanation:
Inorganic compounds like sulphate, phosphate, etc. are commonly found in the acid-soluble pool. Let's understand what the acid-soluble pool is and why these compounds are found in it.

Acid-soluble pool:
The acid-soluble pool refers to the fraction of inorganic compounds that are soluble in acids. These compounds readily dissolve in acid solutions and are thus easily extractable from a given sample. Acid-soluble compounds are typically present in the soil, rocks, minerals, and other geological formations.

Reason for the presence of sulphate, phosphate, etc. in the acid-soluble pool:
1. Solubility in water: Sulphate (SO4) and phosphate (PO4) ions are highly soluble in water. This solubility allows them to be leached out of rocks and minerals by water, and subsequently transported to various natural bodies like rivers, lakes, and oceans.

2. Acid dissolution: When acid is added to a sample containing sulphate or phosphate compounds, these ions readily dissolve in the acid solution. This dissolution occurs due to the reaction between the acid and the compounds, resulting in the formation of soluble salts (e.g., sulphates and phosphates of hydrogen ions).

3. Extraction process: During scientific analysis or experimentation, the acid-soluble pool is often used to extract and quantify certain inorganic compounds. This process involves treating the sample with acid to dissolve the acid-soluble fraction, and then analyzing the resulting solution for the presence and concentration of specific ions.

4. Environmental significance: The presence of sulphate and phosphate ions in the acid-soluble pool is ecologically significant. These ions play crucial roles in various natural processes, such as nutrient cycling, soil fertility, and water quality. For example, phosphate is an essential nutrient for plant growth, while sulphate is involved in microbial metabolism and the formation of important minerals like gypsum.

Therefore, based on the solubility characteristics of sulphate, phosphate, and other inorganic compounds, they are primarily found in the acid-soluble pool. This pool represents the fraction of compounds that readily dissolve in acid solutions and can be easily extracted for analysis or further study.