C. pendant.
Adverbs are invariant words which changes the meaning of a Verb, an Adjective, the sentence itself or a word of the same kind. In this case, we have an adverb that changes the sense of the adjective sérieusement (seriously).
The word vraiment (truly) is used to accompany a calificative Adjective. Ex:
La fête était vraiment amusant. (The party was truly fun)
The word assez (quite) is used to change the sense of a Verb in past Tense, usually applied for contrast purposes. Ex:
La Banque n'a pas considéré assez sérieusement le bien-être économique. (The Bank has not considered quite seriously the economic well-being)
The word pendant (during) is an Adverb related with time, represented by a Substantive. Ex:
pendant cette période. (during that period)
And the word très is an Adverb with a superlative function which changes the sense of another Adverb or Adjective. Ex:
Nous sommes très préoccupés. (We are very concerned)
Hence, we conclude that the only adverb that cannot be used to fill in the blank in the sentence is pendant.
Related questions:
https://brainly.com/question/1059658
https://brainly.com/question/23653991
