Answer: To form the comparative degree of adverbs, the words (C.) more and less should be added.
Explanation: Adverbs are words that are used to modify verbs, adjectives or other adverbs. Moreover, there are different types of adverbs, such as adverbs of time, adverbs of condition, adverbs of place and adverbs of degree. In some cases, to form the comparative degree of adverbs, it is enough adding the suffix -er at the end of the word, such as in "faster" or "quicker"; however, in other cases, it is necessary to add the words "more" or "less" before the adverb in order to form the comparative degree. These two words are generally added when the adverb ends in -ly.