Object + verb [(to be) +pp] + agent Subject + verb + object.
01) The writer gives importance to the action more than the subject; this is different from the newly written sentences which focus on the subject not the action.
02) Matching the sentences with their corresponding tenses:
Sentence (1) c) the present simple passive. Sentence (2) d) the past simple passive. Sentence (3) a) the future simple passive. Sentence (4) b) the present perfect passive.
03) No, it is not always necessary to use ‘by’ in the passive sentences. Because we are supposed to be more interested in the object, not the subject.