Question Detail

Don't touch the fence.

Let the fence not be touch.
Let the fence not be touched.
Let the fence not touched.
Let the fence to not be touched.

Answer: Option B

1. They had not done their tasks.

Their tasks had not been done.
Their tasks had been not done.
Their tasks have not been done.
Their tasks have been not done.

Answer: Option A

2. I must help him.

He must helped by me.
I must held him.
He must get help from me.
He must be helped by me.

Answer: Option D

Explanation:

If we have "Subject + Modal Auxiliary verb + first verb + object"
then we change it to

Passive subject + Modal Auxiliary verb + be + third form + by + passive object.

Modal Auxiliary verb are : can, could, shall, will, may, might, should, would, must, ought to etc.

3. Someone has stolen his book.

His book is been stolen.
His book has been stolen.
His book is stolen.
His book has stolen.

Answer: Option B

Explanation:

Apart from Rule to make active voice of Present Perfect Tense, which is

Passive subject + has/have + been + third form of verb + by + passive object

Please also note that words like, "someone", "somebody", "people", "police", "public" etc are dropped. But in rare cases we can use them if sentence meaning is not clear.