Absolute zero is -

Options

A. `-273^(@)C`
B. Zero K
C. Temperature at which no substance exists in gaseous state
D. All of these

Answer

**Step-by-Step Solution:**

1. **Definition of Absolute Zero**: Absolute zero is defined as the lowest possible temperature where all molecular motion stops. It is the point at which a thermodynamic system has minimal energy.

2. **Temperature Scale**: Absolute zero is measured in Kelvin (K). The absolute zero temperature is 0 K.

3. **Conversion to Celsius**: To convert Kelvin to Celsius, we use the formula:
\[ \text{Temperature in Celsius} = \text{Temperature in Kelvin} - 273.15 \]
Therefore, at 0 K, the temperature in Celsius is:
\[ 0 K - 273.15 = -273.15^\circ C \]
For simplicity, it is often rounded to -273°C.

4. **Third Law of Thermodynamics**: According to the third law of thermodynamics, as the temperature approaches absolute zero, the entropy of a perfect crystal approaches zero. This means that at absolute zero, a perfect crystalline structure would exist, and no substance can exist in a gaseous state.

5. **Conclusion**: Thus, absolute zero is defined as 0 K or -273.15°C, and it is the temperature at which no substance exists in a gaseous state.

**Final Answer**: Absolute zero is 0 Kelvin (0 K) or -273.15°C.

---