The incorrect statemetns amgong the following is

Options

A. The entropy of the universe remains constant
B. Heat cannot be completely converted into work
C. The absoltute entropy of a perferctly cystaline solid at absolute zero temperature is zero
D. The total energy of an isolates system remains contant

Answer

To determine the incorrect statement among the given options, we need to analyze each statement based on the principles of thermodynamics.

### Step-by-Step Solution:

1. **Understanding Entropy**:
- The first statement claims that "The entropy of a unit remains constant."
- Entropy (S) is a measure of disorder or randomness in a system. According to the second law of thermodynamics, the total entropy of an isolated system can never decrease over time. In fact, for any spontaneous process, the total entropy of the universe (system + surroundings) increases. Therefore, the claim that entropy remains constant is incorrect.

2. **Heat and Work Conversion**:
- The second statement states that "Heat cannot be completely converted into work."
- This is a correct statement as per the second law of thermodynamics. It states that not all heat energy can be converted into work due to inherent inefficiencies and losses in the process. Thus, this statement is true.

3. **Absolute Entropy at Absolute Zero**:
- The third statement asserts that "The absolute entropy of a perfectly crystalline solid at absolute zero temperature is zero."
- This is in accordance with the third law of thermodynamics, which states that the entropy of a perfect crystal approaches zero as the temperature approaches absolute zero. Hence, this statement is also true.

4. **Total Energy of an Isolated System**:
- The fourth statement claims that "The total energy of an isolated system remains constant."
- This is a reflection of the law of conservation of energy, which states that energy cannot be created or destroyed, only transformed. Therefore, this statement is also true.

### Conclusion:
After analyzing all the statements, the first statement is the incorrect one. Therefore, the answer to the question is that the incorrect statement is:

**Answer: The entropy of a unit remains constant.**