The CORRECT answer is 4.

The second sentence should be E as it reiterates the same fact as sentence A. The sentence that should succeed E is B. We can confirm the fact from the usage of the word Gurukulam in the third sentence. The word Gurukulam first appears in the second sentence E and the third sentence, B traces back the history of Gurukulam.

The sentence that should succeed the third sentence (B), is F. We can confirm it because, in sentence F, the fact regarding the initiation of Gurukulam (Vedic AGE) reappears, after appearing for the first time in sentence B, the sentence which PRECEDES F. THEREFORE, we can conclude that F should be the fourth sentence. 

The sentence which should succeed the fourth sentence (F), is D because we have already seen that F talks about the chief purpose of Gurukulam i.e training the young minds. And D should succeed F because D further explains in detail, the various domains in which the students were usually trained in.