First set contains three letters B, K, T
Position of B in English alphabet is 2
Position of K in English alphabet is 11=1+1=2
Position of T in English alphabet is 20=2+0=2
Thus all the alphabets of a set have similar positional value.
Similarly for other sets
Now out of ten choices only (I, R) has the same concept
Position of I in English alphabet is 9
Position of R in English alphabet is 18=1+8=9