The correct answers are: The priceless diamond was stolen just before midnight. The security cameras were being repaired during the robbery. The glass display case was smashed into a thousand pieces.

The past passive is formed using was/were + the past participle (e.g., was stolen, was smashed). For the past continuous passive, we use was/were being + the past participle (e.g., were being repaired).

The option about the security guards is incorrect because "guards" is plural and requires "were," not "was."

The option about the alarm system is incorrect because it uses the base verb "disable" instead of the past participle "disabled."