During the quality check last month a group of experts declared much o

During the quality check last month a group of experts declared much of the fire-fighting equipment dysfunctional that was assembled by new people on the job.

If we do sequencing of events - assembling of equipment by new people happened first and the 'quality check' by a group of experts happened later and in that check - the experts declared much of the FFE dysfunctional.

A. During the quality check last month a group of experts declared much of the fire-fighting equipment to be dysfunctional that was assembled.... - here the Rel Clause 'that was assembled by the new people on the job' modifies the equipment but it jumps over verb to be - state of being/condition - So this seems to be incorrect - but lets keep it for now.

B. During the quality check much of the fire-fighting equipment that was declared as dysfunctional by a group of experts had been assembled last month
Incorrect - during the quality check much of the FFE..... had been assembled last month by... were the equipment assembled during the quality check?

C. Much of the fire-fighting equipment that was declared dysfunctional by a group of experts during the quality check was assembled last month
Incorrect - when did the quality check take place ? - not clear
also, C implies the equipment were assembled last month.

D. Much of the fire-fighting equipment that was declared to be dysfunctional by a group of experts during the quality check last month has been assembled
Incorrect - usage of present perfect tense in Passive form 'has been assembled'
E. Much of the fire-fighting equipment that was declared dysfunctional by a group of experts during the quality check last month had been assembled
Correct - Clear and Concise construction with proper sequencing of events and correct usage of past perfect tense in the end of the sentence.

Answer E