Use structure “By + question word + subject + auxiliary + past participle”
As per the given above sentence is given in Passive Voice.
By whom is this mess being created?

Correct Option: D

Use structure “By + question word + subject + auxiliary + past participle”
As per the given above sentence is given in Passive Voice.
By whom is this mess being created?
Hence , option D is correct sentence .

Use structure “subject + auxiliary + past participle + by + object”
As per the given above sentence is given in Passive Voice.
A perfect dam has been built by them across the river.

Correct Option: B

Use structure “subject + auxiliary + past participle + by + object”
As per the given above sentence is given in Passive Voice.
A perfect dam has been built by them across the river.
Hence , required answer is option B .

We can say that ,
The above given sentence is given in Passive Voice :-
Use structure “subject + auxiliary + past participle + by + object”
The instructions were being read out by the invigilator.

Correct Option: B

We can say that ,
The above given sentence is given in Passive Voice :-
Use structure “subject + auxiliary + past participle + by + object”
The instructions were being read out by the invigilator.
Hence , required sentence is option B .

Use structure “subject + past participle + object”
As per the given above sentence is given in Passive Voice.
The flattery of his assistant disgusted him.

Correct Option: A

Use structure “subject + past participle + object”
As per the given above sentence is given in Passive Voice.
The flattery of his assistant disgusted him.
Therefore , required answer will be option A .

We can say that ,
The above given sentence is given in Passive Voice :-
Use structure “By + whom(Question word) + auxiliary + subject + be + past participle + ?”
By whom can the stars in the sky be counted?

Correct Option: A

We can say that ,
The above given sentence is given in Passive Voice :-
Use structure “By + whom(Question word) + auxiliary + subject + be + past participle + ?”
By whom can the stars in the sky be counted?
Hence , required sentence is option A .