C) finally
The word "finally" indicates that the speaker is about to reveal the next action or step that will follow. In this context, the speaker is concluding their congratulations and wishes with an invitation to the theater. The words "in the end" and "at last" indicate more of a sense of relief, as if something that was long-awaited has finally come to fruition. However, there is no indication that this invitation was a long-awaited event. Therefore, "finally" is the most appropriate choice.