Clearly, the first letters of the first, second, third, fourth and fifth terms are moved three, four, five, six and seven steps forward respectively to obtain the first letter of the successive terms.
The second letters of the first, second, third, fourth and fifth terms are moved five, six, seven, eight and nine steps forward respectively to obtain the second letter of the successive terms.
The third letters of the first, second, third, fourth and fifth  terms are moved seven, eight, nine, ten and eleven steps forward respectively to obtain the third letter of the successive terms.
Thus, the missing term would be ZKW.
Hence, the answer is (a).