Read the first paragraph carefully. The two contentions are based on opposing interpretation of the role trade unions played after WWII. In the last line of the 1st para the author says that historical reality cannot be reduced to both since it isn't easy to reconcile the two arguments. Thus the passage is concerned more with evaluating these two viewpoints. The whole of the 2nd para simply provides a context in which to evaluate the viewpoints.

The first paragraph states two opinions - 1. the emergence of Afro-American alliance [suggesting some sort of equality] and 2. the contention that Whites were superior [def no equality here!] to Africans. Obviously these two opinions are contradictory. Which is stated in the closing sentence of the first paragraph.
The quickest way would be to look for the option which mentions something related to 2 opposing views. only A does that. All the others seem to focus only on one theory, one opinion or one phenomena.

To be doubly sure of your answer its best to summarize what para 2 talks about in relation to para1. That would clearly suggest A - a situation which helps evaluate the 2 irreconcilable opinions.

hope this helps.