The teachers informed the students that the class had been cancelled.
Right Answer is: C
The given sentence is in active form of simple past tense. The structures for active/passive voices are:
Active: Subject + verb (IInd form) + object…
Passive: Object + was/were + verb (IIIrd form) + by + subject…
So, with the help of the above structures, we can convert the given sentence into passive voice:
The students were informed by the teachers that the class had been cancelled.