The appropriate verb form must express the past tense, as mentioned by the verb phrase 'did he do'.
Option A: 'Was Bill sacked' expresses the simple past tense in the interrogative and in passive voice. It's the most appropriate option here, since it expresses the right tense and voice. 'Was sacked' means that Bill receives the action 'to be dismissed from his job' ('sacked' means 'dismissed'). Hence option A is right.
Option B: 'Bill sacked' is in active voice and means that 'Bill sacked someone else'. But this action has no recipient. There has to be an object of the action in the active voice here.
Option C: 'Is Bill sacked' is in the simple present tense. This tense expresses a daily habit or fact. But the action here has already happened.
Option D: The clause 'Bill was sacked' is declarative, as opposed to the interrogative sentence given 'Why...?'. So, it's not suitable.
Hence options B, C and D are wrong.