Hatch and Slack pathway (HSK pathway) is otherwise known as `C_(4)` -cycle because -

Options

A. The first stable product is oxaloacetic acid OAA which is a `C_(4)` -compound
B. The primary `CO_(2)` acceptor is OAA, a `C_(4)` compound
C. All intermediate metabolites are `C_(4)`-compounds
D. At one time `4CO_(2)` molecules take part in carboxylation pathway

Answer

### Step-by-Step Solution:

1. **Understanding the Hatch and Slack Pathway**: The Hatch and Slack pathway, also known as the C4 cycle, is a process of photosynthesis that occurs in certain plants, referred to as C4 plants.

2. **Identifying the First Stable Product**: In the C4 cycle, the first stable product formed during the fixation of carbon dioxide is oxaloacetic acid (OAA), which is a 4-carbon compound.

3. **Role of the Primary CO2 Acceptor**: The primary CO2 acceptor in this pathway is phosphoenolpyruvate (PEP), which combines with CO2 to form OAA. This emphasizes that the cycle is based on the incorporation of carbon dioxide into a 4-carbon compound.

4. **Characteristics of C4 Plants**: C4 plants, such as maize and sugarcane, exhibit a specialized leaf anatomy known as Kranz anatomy. This includes bundle sheath cells that surround vascular bundles and are involved in the C4 pathway.

5. **Formation and Transfer of Compounds**: The C4 acid (OAA) is formed in the mesophyll cells and then transported to the bundle sheath cells, where it is converted into a 3-carbon compound (C3 acid), which is then cycled back to the mesophyll cells.

6. **Conclusion**: The Hatch and Slack pathway is referred to as the C4 cycle because the first stable product of this pathway is oxaloacetic acid, which is a 4-carbon compound.