In the following question, select the related number pair from the given alternatives:

15 : 256 :: 18 : ?

361

The correct answer is Option (2) → 361

15 : 256 :: 18 : ?

THE LOGIC FOLLOWED HERE IS

(IST NUMBER + 1)² = IIND NUMBER

AS

(15 + 1)² = IIND NUMBER

(16)² = IIND NUMBER

256 = IIND NUMBER 

SIMILARLY

18 : ?

(18 + 1)² = IIND NUMBER

(19)² = IIND NUMBER

361 = IIND NUMBER