Which of the compounds is known as Slaked lime?

The correct option is Ca(OH)₂.

To understand why calcium hydroxide, or Ca(OH)₂, is known as slaked lime, we can look at its chemical formation and common nomenclature:
1. Quicklime: Calcium oxide, or CaO, is commonly referred to as quicklime or burnt lime. It is produced by heating calcium carbonate (limestone).
2. The Slaking Process: When water is added to quicklime (calcium oxide), a highly exothermic chemical reaction occurs. This process is called "slaking." The reaction is represented by the following chemical equation:

$$\text{CaO}+{\text{H}}_2\text{O}\to \text{Ca}{\left(\text{OH}\right)}_2$$

3. Slaked Lime: The product of this slaking reaction is calcium hydroxide, Ca(OH)₂. Therefore, it is commonly known as slaked lime.

Let's also review the other options for clarity:
- CaO: Known as quicklime or calcium oxide.
- CaSO₄: Known as calcium sulfate, which in its dihydrate form (CaSO₄·2H₂O) is gypsum.
- CaCO₃: Known as calcium carbonate, which is the primary constituent of limestone, chalk, and marble.