The correct option is C O,U
Clearly, the first, second, third, ..... letters of the series are respectively moved two, three, four, ..... step forward to obtain the successive terms of the series.
Thus, the fifth term in the series must be a letter which is five steps ahead of J i.e. O, while the sixth term must be a letter six steps ahead of O i.e. U.
Thus, we have the following pattern
A+2−→C+3−→F+4−→J+5−→O+6−→U
So, the missing terms are O and U. Hence, the answer is (C).