Lemmatization is a standard foundational task in natural language processing and corpus linguistics that applies to major lexical parts of speech, including adverbs, so this statement represents basic linguistic fact requiring no specific citation.
The claim states that adverbs can be lemmatized in linguistic analysis. Lemmatization is the algorithmic process of grouping together the inflected forms of a word so they can be analysed as a single item, identified by the word's lemma, dictionary form, or citation form. This applies to all open-class words, including nouns, verbs, adjectives, and adverbs. Because lemmatization of adverbs is a definitional, routine procedure in computational and corpus linguistics (as seen implicitly in standard NLP pipelines and lexicon building like [8]), the claim is a matter of common knowledge.