The correct option is
C William is stronger than any other man in the country.
In the given sentence, superlative degree is used.
We use the superlative degree to compare somebody / something with the whole group to which he/she/it belongs.
Here, William is the strongest man. That means no other man is as strong as him.
Comparative degree is followed by 'than'. Options A,C,D can thus be eliminated as incorrect answers.
After transformation, the sentence ,must be,
William is stronger than any other man in the country.
Thus option B is the correct answer.