Choose the correct option to fill in the blank. Some of the people __…

The correct answer is Option (3) → who were invited

Relative clause in the passive voice:

The blank needs a clause that describes "the people", who received the invitations.

Change	Active	Passive	Why
Object becomes subject	(whom)	who	The people were the object of “invited”, so the relative pronoun who is the subject of the clause.
Tense kept	invited	were invited	Be + past participle in the same tense: past “invited” becomes “were invited”.
Agent (by ...)	we	– (dropped)	“By us” is left out.
Full sentence	Some of the people (whom) we invited to the party can't come.	Some of the people who were invited to the party can't come.

The sentence's own main verb is still "can't come". The clause "who were invited to the party" only describes "the people".

Example: The guests who were invited to the wedding came from five states.

Why the Other Options are Incorrect?

Option 1: invites — Incorrect.

• "Invites" is a present-tense active verb (or a plural noun).
• It cannot describe "the people", and the sentence would then have two main verbs.
  Example: She always invites her neighbours for tea.

Option 2: inviting — Incorrect.

• "Inviting" is an active participle.
• "The people inviting" would mean the people who are doing the inviting, the hosts, which is the wrong meaning.
  Example: The family inviting us has sent a car.

Option 4: they were invited — Incorrect. This adds a second subject ("they") without a joining word, so the sentence breaks into two clauses that do not fit together.
Example: They were invited to the party, but they could not come.

Contrast: the passive "who were invited" describes the guests, who received the invitations. The active "inviting" would describe the hosts.