Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
_ msk _ prm _ kt _ r _ s _ tpr _ skt _
1
r, s, t, p, m, k, m, p
2
r, t, s, p, m, m, m, p
3
r, t, s, p, m, k, k, p
4
r, t, s, p, m, k, m, p