Assertion: Helium has the highest value of ionisation energy among all known elements.
Reason: Helium has the highest value of elelctron affinity among all known elements.

Options

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

Explanation

To solve the given question, we need to analyze the assertion and reason statements regarding helium.

### Step-by-Step Solution:

1. **Understand the Assertion**:
- The assertion states that "Helium has the highest value of ionization energy among all known elements."
- This is true. Helium, with an electronic configuration of 1s², has a very high ionization energy due to its small atomic size and the strong attraction between the nucleus and the electrons.

2. **Understand the Reason**:
- The reason states that "Helium has the highest value of electron affinity among all known elements."
- This is false. Helium is a noble gas, and noble gases have very low electron affinity because they have a complete valence shell. Helium's electron affinity is effectively zero, as it does not readily accept an electron.

3. **Evaluate the Relationship**:
- The assertion is true, but the reason is false. Therefore, the reason does not correctly explain the assertion.

4. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false.

### Final Answer:
The correct option is C: "Assertion is true but reason is false."

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