Clearly, the first letter of each term is moved two steps backward to obtain the first letter of the next term.
So, the first letter of  the missing term will be Q.
The second letter of the first, second, third, fourth terms are respectively moved one, two, three and four steps forward to obtine the corresponding letter of the subsequent term.
So, the second letter in the missing term will be O.
The third letter is alternately moved two and three steps forward to obtain the corresponding letter of the subsequent term.
So, the third letter in the missing term will be L.
Thus, the missing term is QOL.
Hence, the answer is (a).