Select the option that is related to the third number in the same way as the second number is related to the first number. 54 : 81 : : 6 : ?

21 9 15 11

9

THE LOGIC FOLLOWED HERE IS : ( IST NUMBER  ÷ 6) × 9 = IIND NUMBER AS ( 54  ÷ 6) × 9 = 9 × 9 = 81 SIMILARLY 6 : ? ( 6  ÷ 6) × 9 = 1 × 9 = 9