Step 1: Understanding the Concept:
This is a prepositional collocation problem in English, where certain adjectives must be followed by specific prepositions to be grammatically correct.
Step 2: Detailed Explanation:
- The adjective "nostalgic" refers to a sentimental longing or affectionate wistfulness for a period in the past.
- In standard English grammar, the adjective "nostalgic" is paired with the preposition "about" when referring to the subject of nostalgia (e.g., "nostalgic about the past", "nostalgic about old memories").
- Using "with", "to", or "at" after nostalgic is unidiomatic and incorrect.
- Therefore, the correct sentence is: "He was nostalgic about his childhood."
Step 3: Final Answer:
The correct option is (C) About his.