The given sentence 'they were the honoured guests...' narrates a past event. 'Were' is a past tense verb.
Option B: 'Forbade' is a past tense verb that means 'to stop (with authority) someone from doing something'. It agrees to the given sentence which narrates a past event. Hence, option B is correct.
Option A: 'Has forbidden' is in the present perfect tense. It expresses an action that began in the past and has just completed in the present. The given sentence states an action that completed in the past. Hence, option A is wrong.
Option C: 'Forbidden' on its own is an adjective. To refer to an action, it requires a helping verb (auxiliary verb) like 'has', 'had', etc. The sentence requires an action word and not an adjective. Hence, this option isn't suitable.
Option D: 'Was forbidding' is in the past continuous tense. It expresses a continuous action that happened in the past. But the action here 'to forbid' isn't meant to be continuous. It's a short action that completed in the past. Hence, option D is wrong.