Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the series.

r_s_p_rqs_prrqs_pr

qsrss

After dividing in three groups of six each we get  r_s_p_rqs_prrqs_pr  After comparing different positions we get as a pattern.

rqsspr / rqsspr / rqsspr

So the group having letters qsrss will be used.