The passage suggests that one of the "assumptions of the 1950s” regarding the meaning of a political text was that this meaning:
could be definitively established without reference to the text's historical background
Look at the line “assumptions of the 1950s, when it was naively thought that the close reading of a text by an analytic philosopher was sufficient to establish its meaning, even if the philosopher had no knowledge of the period of the text’s composition”. From this we can infer that during 1950s, it was thought that the meaning of a text can definitely be established even without reference to the text’s historical background. Hence option B.