The pronoun which correctly completes the sentence is the following one:
D- ourselves.
The complete sentence would look like this:
The only students invited to attend the meeting were ourselves.
"Ourselves" is a reflexive pronoun, and it fits the sentence above because it can be used as the subject of the passive sentence. "We" could also have been used as a subject, but then the sentence would have to be in a different order, as in: "We were the only students invited to attend the meeting".
"Them" and "us" must be ruled out, since they are object pronouns.