Look at the transition word ‘but’ in the first sentence.
It signifies that the sentence is expressing an idea contrary an idea expressed in some previous sentence. Now we need to find that previous sentence. If we further look at the beginning of the first sentence, it says ‘but in the institutional era’ which suggests that the contrariness is with respect to eras.
Looking further, we see that sentence 2 and 3 are also starting with statement about eras. But the transition word at the start of 3 is ‘now’ which expresses present era and hence it cannot chronologically come before any other past era. That is, if information era is the present era, talk about any other era will come before this.
So sentence 2 is the correct sentence to come before the first sentence. Likewise, sentence 3 is the correct sentence to come after the first sentence (sentence 3 is continuing the idea).
Therefore, we have the link 213. We see that option A, C and D all have the link 213.
Furthermore, all the three options have the link 5213. Therefore, we only need to arrange 4 and 6.
The sentence 6 states that ‘The purpose is to battle the enemy’ and 4 questions 'how do you battle the enemy'
Therefore 4 will come after 5.
Hence option A is the correct answer