Correct option is (b) BIDNHTSSX
The pattern followed here is:-
For the letters in odd places(1st, 3rd, 5th, 7th, 9th,11th) Positional value of the letter - 1
For the letters in even places(2nd, 4th, 6th, 8th,10th) Positional value of the letter + 1

Hence, the code for "CHEMISTRY" is "BIDNHTSSX".