The correct option is C was
In this sentence, 'the plane' is the subject. It is a singular noun. As the subject is singular, the verb must also be singular.
The clause "with all its passengers" provides additional information but it does not have any other function in the sentence. Hence the verb is not affected by the phrase.
Options A and C: 'Were' should be replaced with 'was', because the latter is a singular verb and agrees to the subject, which is singular.
The verb must agree with the subject in terms of number (single/plural) and person (1st, 2nd, 3rd person).
Hence C is correct and A is incorrect.
Option B: 'Are' is a plural verb, so this option is incorrect.
Option D: 'Were to be' means that something was to have happened, but it didn't. This does not complete the sentence in meaning. Hence D is incorrect.