Explanation:

- Subject-Verb Agreement: When using "neither/nor," the verb should agree with the noun or pronoun closest to it.

- Breakdown:

- "Neither you / (B) nor she / (C) is happy." The verb "is" agrees with "she", the pronoun closest to the verb.

- Option 1 (a) checks the part "Neither you". It's correct.

- Option 2 (b) checks the part "nor she". It's correct.

- Option 3 (c) checks the verb "is happy". It's correct because it agrees with "she".

- Option 4 (d) implies no error in the sentence.

- Conclusion:

- Correct Answer: Option 4 (d). The sentence is grammatically correct as it stands.