Option 1 -> "invites" is a verb form that doesn't grammatically fit in this sentence structure.
Option 2 -> "inviting" would incorrectly suggest the people are doing the inviting, and "inviting to" is not proper usage.
Option 3 -> "who were invited" correctly forms a relative clause in passive voice to modify "people."
Option 4 -> "they were invited" creates a grammatically incorrect sentence with an improper clause structure.
Hence, Option 3: who were invited -> This creates a proper relative clause that modifies "people" and uses the passive voice to indicate that the people received invitations. The complete sentence reads: "Some of the people who were invited to the party can't come." -> correct