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

Let's check the total number of characters including blanks:
The given sequence is: _ w w _ x w _ x x _ w x _ w w x (Total 16 places).

Let's divide the series into 4 groups of 4 letters each:
_ w w _ | x w _ x | x _ w x | _ w w x

Let's test the repeating pattern of x w w x:

First group: Put x at 1
st
and x at 4
th
position → x w w x

Second group: Put w at 7
th
position → x w w x

Third group: Put w at 10
th
position → x w w x

Fourth group: Put x at 13
th
position → x w w x

The complete filled sequence becomes:
x w w x | x w w x | x w w x | x w w x

The filled letters in sequential blank order are: x, x, w, w, x