Which of the following is a true statement

Options

A. The total entropy of thermally interacting systems is conserved
B. Camot engine has 100% efficiency
C. Total entropy does not change in a reversible process
D. Total entropy in an irreversible process can either increase or decrease

Explanation

To determine which statement is true regarding thermodynamics and entropy, let's analyze each option step by step:

### Step 1: Evaluate Option 1
**Statement:** The total entropy of the thermally interacting system is conserved.

**Analysis:**
- The total entropy of a system can change when heat is exchanged between systems. According to the second law of thermodynamics, the total entropy of the universe (system + surroundings) tends to increase over time. Therefore, this statement is false.

### Step 2: Evaluate Option 2
**Statement:** A Carnot engine can have 100% efficiency.

**Analysis:**
- A Carnot engine operates between two heat reservoirs and achieves maximum efficiency. However, it cannot achieve 100% efficiency because some energy is always lost as waste heat to the cold reservoir. Thus, this statement is also false.

### Step 3: Evaluate Option 3
**Statement:** Total entropy does not change in a reversible process.

**Analysis:**
- In a reversible process, the total change in entropy of the system and surroundings is zero. This means that the entropy of the system can change, but the total entropy change (system + surroundings) remains constant. Therefore, this statement is true.

### Step 4: Evaluate Option 4
**Statement:** Total entropy in an irreversible process can either increase or decrease.

**Analysis:**
- In an irreversible process, the total entropy of the universe always increases (ΔS > 0). It cannot decrease. Therefore, this statement is false.

### Conclusion
After evaluating all the options, the only true statement is option 3: "Total entropy does not change in a reversible process."

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