Assertion:Absolute values of intenal energy of substances cannot be determined.
Reason:It is impossible to determine exact values of constituent energies of the substances.

Options

A. If both the assertion and reason are true and reason is correct explanation of the assertion .
B. If both the assertion and reason are true but reason is not a correct explanation of the assertion .
C. If the assertion is true but the reason is false.
D. If both assertion and reason are false.

Explanation

**Step-by-Step Solution:**

1. **Understanding Internal Energy**: Internal energy (U) is a thermodynamic property that represents the total energy contained within a system. It includes various forms of energy such as translational, rotational, vibrational, and potential energies.

2. **Assertion Analysis**: The assertion states that "absolute values of internal energy of substances cannot be determined." This is true because internal energy is a state function, and we can only determine changes in internal energy (ΔU) between two states, not the absolute values of the internal energy itself.

3. **Reason Analysis**: The reason provided states that "it is impossible to determine exact values of constituent energies of the substances." This is also true because the internal energy is a sum of many microscopic energies, and measuring each of these constituent energies accurately is practically impossible.

4. **Connecting Assertion and Reason**: Since both the assertion and reason are true, we need to evaluate whether the reason correctly explains the assertion. The reason does explain why we cannot determine absolute values of internal energy, as it highlights the complexity and the difficulty in measuring the constituent energies.

5. **Conclusion**: Since both the assertion and reason are true, and the reason provides a correct explanation for the assertion, the correct answer is option A: "If both the assertion and reason are true, reason is the correct explanation of the assertion."