If you read each of these, or speak them, there is only one that suits the sentence correctly.
Go with the third one. Couldn't seems to fit pretty well there.
The option that provides the best definition of the term avant-garde as the author intends it in the passage is (b.) innovative.
<em>Avant-garde </em>is a term taken from French that is used to refer to new ideas that are introduced in fields such as music or literature.<u> It is a concept that is mainly connected to artists that are ahead of their time and also to innovative works of art</u>, especially those that were produced during the 20th century. In this case, <u>a</u><u><em>vant-garde</em></u><u> has been used to describe </u><u><em>Waiting for Godot</em></u><u> since the play appeared to be radically original when it was premiered due to its characters, settings and themes</u>.