Explanation:

- The sentence refers to a specific moment when the child saw his parents and describes his emotional reaction.

- Option 1: Where

- "Where" refers to a location.

- It doesn't fit as the sentence talks about timing rather than place.

- Option 2: Who

- "Who" refers to a person.

- It doesn't suit the context since the sentence isn't identifying someone.

- Option 3: When

- "When" refers to time and is appropriate here.

- It fits the context as it shows a timing relationship between two events: the child seeing his parents and becoming happy.

- Option 4: While

- "While" suggests simultaneous actions.

- However, it doesn't convey the sequential or causal relationship clearly.