Select the set in which the numbers are related in the same way as are the numbers of the following sets.
(Note: Operations should be performed on the whole numbers, without breaking down the numbers into their constituent digits. E.g., 13 – Operations on 13 such as adding/subtracting/multiplying to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed.)
(19, 31, 55)
(14, 26, 50)

(19, 31, 55)
(14, 26, 50)

Logic:
In each set, the pattern is:

Second number = First number + 12
Third number = Second number + 24

(19, 31, 55)
19 + 12 = 31
31 + 24 = 55

14 + 12 = 26
26 + 24 = 50

A) (48, 60, 84)
48 + 12 = 60
60 + 24 = 84 → follows 

B) (18, 32, 54)
18 + 14 ≠ 32 → does not follow

C) (15, 30, 45)
15 + 15 ≠ 30 → does not follow

D) (13, 26, 43)
13 + 13 ≠ 26 → does not follow

Thus, correct option is (a).