If,

A C D; B C D; B K M; C O R and D D H is the given sequence, then if the number of each letter in the alphabet is replaced in each equation it reads like 1 3 4; 2 5 7; 2 11 13 and so on. It can then be found a sum of first two numbers is the third number. With this in mind, if you write the last equation with its numerical value, you get the first two numbers as L (12) & N (14). Therefore the third number is sum of 12 and 14 that is 26 which is represented by a letter Z. Hence ?= Z.