Answer:
Claire is much <em><u>more clever/ cleverer</u></em> than her sister Charlotte.
Explanation:
The comparative form of the verb is the use of comparison in a verb. And in the case of the word "clever", there are two ways of comparison- more clever and cleverer.
Now, in the given sentence<em> "Claire is much .... than her sister Charlotte."</em> The comparative "much" is already used in the sentence so we can either add "more clever" or just simply "cleverer" to add/ emphasize Claire.
So, the final sentence will be<em> "Claire is much </em><u><em>cleverer/ more clever </em></u><em>than her sister Charlotte."
</em> Either way, the use of both is correct.
