What word correctly fills in the gap regarding the wedding invitation?

The word that correctly fills in the gap regarding the wedding invitation is "haven't." This choice is appropriate because it fits the context of a wedding invitation, which typically conveys a sense of anticipation or a missing element—such as information about the event or a request for attendance.

Using "haven't" suggests that something is still pending or expected, aligning well with the conventions of invitations that invite guests to share in a joyous occasion, making it more engaging and personal. It indicates a current state of affairs in relation to the guest's involvement or knowledge.

The other options do not work as effectively in this context. "Didn't" refers to something that occurred in the past and doesn’t capture the ongoing nature of planning or awaiting the event. "Weren't" is also past tense and would be suitable in a different situation, perhaps referring to people not being present at a past event. "Isn't" implies a present state of non-existence but does not convey the anticipation or invitation aspect as well as "haven't" does.