Yes, you are correct. The correct answer is D, adding the suffix "est" to the adverb widely to form the superlative degree. To explain this further, to form the superlative degree of most adverbs, you usually add the suffix "-est" after removing the "ly" ending. So in this case, by adding "-est" to the stem word wide, it becomes widest, making it the superlative form of the adverb widely.