Answer: more or less.
Comparative forms of both irregular adjectives and regular adverbs is formed by adding words more (if the comparison is positive) or less (if the comparison is negative). For example:
He finished his work more carefully than I did.
As you can see here, the adverb carefully is combined with more to form its comparative form. If you want to make a superlative, you will use most or least.
Explanation:
