Correct option is D
Logic - three times of first number + three times of third number = second number
(12,51,5) = 36 + 15 = 51
(5,36,7) = 15 + 21 = 36
similarly in option d (11,51,6)
triple of first number 11 =33
triple of third number 6 = 18
second number = 33 + 18 = 51