Step 1: Interpreting the original sentence.
The sentence describes two actions in sequence:
First, he finished his exercise.
Second, he put his books away.
Step 2: Applying the instruction.
We must begin the sentence with Having.
The structure "Having + past participle" is used to show that one action was completed before another.
Step 3: Analyzing the options.
(A) alters the intended meaning and structure.
(B) reverses the order of actions.
(C) correctly shows that finishing the exercise happened first, followed by putting the books away.
(D) alters the intended meaning completely.
Step 4: Conclusion.
Hence, the correct rewritten sentence is option (C).