Answer: Curt gave his speech most convincingly of all the class.
Explanation:
There are three degrees of comparison in adverbs and adjectives: the positive, comparative and superlative.
Convincingly is an adverb. Its meaning is - in a way which makes others believe that something is true or real. As the word convincingly ends in -ly, we add 'more' before the word to form the comparative (more convincingly), and 'most' to form the superlative (most convincingly). In this case, the phrase 'of all the class' indicates that we need the superlative form, because we want to emphasize that Curt's speech was the most convincing speech in a whole class.
