1) Why were the trousers not washed before they were hung? WHY + FORM OF BE + SUBJECT + PP + BEFORE + PRONOUN + FORM OF BE + PP 2) By whom were you given my telephone number? BY + WHOM + FORM OF BE + PRONOUN + PP + PRONOUN + SUBJECT 3) That novel was written by Charles Dickens, wasn't it. SUBJECT + FORM OF BE + PP + AGENT + COMMA + QUESTION TAG 4) Has the door been seen to yet? AUXILIARY + SUBJECT + PRESENT PERFECT PASSIVE + PP + PREPOSITION + ADVERB 5) Who are the visiting delegations going to be welcomed by? WHO + FORM OF BE + SUBJECT + GOING TO PASSIVE + PP + BY 6) Will the project have been completed by the end of the year? AUXILIARY + SUBJECT + FUTURE PERFECT PASSIVE + PP + TIME PP => PAST PARTICIPLE
We don't omit BY in passive questions with WHICH, WHO, WHOM.