Note :- Rule to make active voice of Past Perfect Tense :
Passive subject + had + been + third form of verb + by + passive object
3. He found the student guilty.
The student was found to be guilty by him.
The student was found guilty by him.
The student was found to guilty by him.
The student found to be guilty by him.
Answer: Option A
4. 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.