Explanation:

In D: 25, Value of D in English alphabet is ‘4’ and next term is obtained by (1 + 4)2 ⇒ 25
Similarly, F is 6th term in alphabet so next term will be found by (1 + 6)2 = 72 = 49
So, D : 25 :: F : 49.

Hence, option 2 is the correct answer.