If we give number to each letter according to its alphabetical order we find that,
(1) KJA ⇒ K – J = 11 – 10 = 1 = A (2) UTA ⇒ U - T = 21 – 20 = 1 = A (3) OMA ⇒ O – M = 15 – 13 = 2 = B (4) LKA ⇒ L – K = 12 – 11 = 1 = A. Therefore, OMA is the different one.