Select the combination of letters that when sequentially placed in the blanks of the given series, will complete the series.
sr_ss_s_s_ss

The given series has 12 characters: sr_ss_s_s_ss.
Let's see if we can form a repeating pattern by testing the options:

Let's test Option D (rrss):
sr[r]ss[r]s[s]s[s]ss
Let's write it down:
s r r s s r s s s s s s -> no clear small pattern.

Let's test Option C (srsr):
sr[s]ss[r]s[s]s[r]ss
String: s r s s s r s s s r s s
Let's divide into groups of 4:
s r s s | s r s s | s r s s
This gives a perfect repeating pattern of srss.

Therefore, the letters to fill the blanks are s, r, s, r sequentially.