I think the invitations (should / send) by registered post.
should send
should be sent
should sended

I think the invitations (should / send) by registered post.

Correct answer: should be sent

Explanation: In this sentence, we are talking about what should be done with the invitations. "Should" is a modal verb used to express an obligation or a recommendation. When using "should" with another verb, we need to use the base form of the verb, which is "send" in this case.

However, since we are talking about the invitations being sent, we need to use the passive voice. The passive voice is formed using a form of the verb "be" + past participle. In this case, "sent" is the past participle form of the verb "send". So the correct sentence would be "I think the invitations should be sent by registered post."

The other options are incorrect:
- "should send" is not the correct form of the verb because we need to use the passive voice in this sentence.
- "should sended" is not a grammatically correct form of the verb. The past participle form of "send" is "sent", not "sended".