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 <em>sérieusement</em> (seriously).
The word <em>vraiment</em> (truly) is used to accompany a <em>calificative</em> Adjective. Ex:
La fête était <em>vraiment</em> amusant. (The party was truly fun)
The word <em>assez</em> (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 <em>assez</em> sérieusement le bien-être économique. (The Bank has not considered quite seriously the economic well-being)
The word <em>pendant</em> (during) is an Adverb related with time, represented by a Substantive. Ex:
<em>pendant</em> cette période. (during that period)
And the word très is an Adverb with a <em>superlative</em> function which changes the sense of another Adverb or Adjective. Ex:
Nous sommes <em>très</em> 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 <em>pendant</em>.
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