Which of the following is commonly known as Quicklime?

CaO

The correct answer is option 4. CaO.

Quicklime is the common name for calcium oxide, which has the chemical formula CaO. Here's a brief explanation:

1. CaO (Calcium Oxide): Quicklime is a white, crystalline solid that is produced by heating calcium carbonate (such as limestone) at high temperatures. The chemical equation for this process is:

\[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]

2. Properties and Usage:  Quicklime is highly reactive with water and produces a strong exothermic reaction, forming calcium hydroxide (\(\text{Ca(OH)}_2\)). This reaction is utilized in various applications, such as in the construction industry for making mortar and plaster. It is also used in the steel industry, water treatment, and as a soil amendment in agriculture.

3. Not the other options:

\(\text{CaSO}_4\) is calcium sulfate (gypsum), not quicklime.

\(\text{CaCl}_2\) is calcium chloride, not quicklime.

\(\text{Ca(OH)}_2\) is calcium hydroxide (slaked lime), which is the result of adding water to quicklime.

So, among the given options, \(\text{CaO}\) is the compound commonly known as quicklime.