Choose an appropriate word that can substitute the bracketed word and complete the sentence meaningfully.
Shelley was very happy (while) I accepted her invitation to her birthday party.

The conjunction "when" is used to denote the specific time or occasion at which an event happens. Shelley's happiness was a result of/triggered at the specific moment the invitation was accepted. "While" implies two actions happening simultaneously over a period of time, which does not fit. Thus, "when" is the most appropriate substitute.