The sentence fits best in Blank 2. Putting it there adds more detail to the timeline and challenges existing scientific views, making the paragraph flow logically.It doesn't belong in Blank 1 because it doesn't add to the previous sentence.
Placing it in Blank 3 or Blank 4 would disrupt the paragraph's flow because of the connection between the sentences before and after those blanks.
Therefore, the correct option is (D) : option 2.
For any natural number $k$, let $a_k = 3^k$. The smallest natural number $m$ for which \[ (a_1)^1 \times (a_2)^2 \times \dots \times (a_{20})^{20} \;<\; a_{21} \times a_{22} \times \dots \times a_{20+m} \] is: