Which number will best complete the relationship given below?
25:9::16:?

Correct option is B

25 is a square of 5 

now logic is = square of  (square root of first number - 2)

5 - 2 = 3 

second number = square of 3 = 9 

similarly,

for 16

second number = square of (square root of 16 - 2)

= square of (4-2)

square of 2

= 4

so option b is correct.