`DeltaS=(q_(rev))/(T)`, so

Options

A. `Delta S` is defined only for reversible process.
B. For irreversible process, `DeltaS` is calculated by considering the irreversible.
C. For irreversible process, `A rarr B` and same process taking place reversible, `DeltaS` is same.
D. `Delta S_(sys)` is always the for irreversible process.

Explanation

To solve the question regarding the change in entropy (ΔS) and its relationship with reversible processes, we will analyze the statements provided and determine their validity step by step.

### Step-by-Step Solution:

1. **Understanding the Definition of ΔS**:
The formula for change in entropy is given by:
\[ \Delta S = \frac{q_{\text{rev}}}{T} \]
where \( q_{\text{rev}} \) is the heat exchanged in a reversible process and \( T \) is the absolute temperature.

**Hint**: Remember that ΔS is defined in terms of reversible heat transfer, but it can also apply to irreversible processes.

2. **Evaluating Statement 1**:
The first statement claims that ΔS is defined only for reversible processes. This is incorrect because while the formula uses reversible heat, ΔS can be calculated for irreversible processes as well.

**Hint**: Consider whether a concept can apply to both reversible and irreversible processes.

3. **Evaluating Statement 2**:
The second statement suggests that for irreversible processes, ΔS is calculated by considering the irreversible heat. This is also incorrect. The change in entropy is the same for both reversible and irreversible processes because it is a state function.

**Hint**: Recall that state functions depend only on the initial and final states, not on the path taken.

4. **Evaluating Statement 3**:
The third statement asserts that for an irreversible process from A to B, the ΔS value is the same as that for a reversible process from A to B. This statement is correct, as we have established that ΔS is a state function.

**Hint**: Think about how state functions behave when comparing different paths between the same two states.

5. **Evaluating Statement 4**:
The fourth statement claims that ΔS of the system is always that for irreversible processes. This is incorrect because ΔS can be defined for both reversible and irreversible processes.

**Hint**: Reflect on the definitions and properties of state functions in thermodynamics.

### Conclusion:
After evaluating all the statements, we conclude that:
- Statement 1 is incorrect.
- Statement 2 is incorrect.
- Statement 3 is correct.
- Statement 4 is incorrect.

Thus, the correct answer is that Statement 3 is true.