Select the combination of letters that when sequentially placed in the blanks of the given series will complete the series.
_ A T _ R L A _ E R L _ _ E _ _ A _ E R

The given series is: _ A T _ R L A _ E R L _ _ E _ _ A _ E R
Let us count the total number of spaces including blanks. There are 20 positions.
Let's divide the series into 4 groups of 5 letters each:

_ A T _ R

L A _ E R

L _ _ E _

_ A _ E R

By comparing the groups:

From group 2 (L A _ E R) and group 4 (_ A _ E R), we can infer that the pattern is L A T E R.

Group 1: _ A T _ R -> fill with L and E to make L A T E R.

Group 2: L A _ E R -> fill with T to make L A T E R.

Group 3: L _ _ E _ -> fill with A, T, R to make L A T E R.

Group 4: _ A _ E R -> fill with L, T to make L A T E R.

Let's check the letters filled in order:

1st blank: L

2nd blank: E

3rd blank: T

4th blank: A

5th blank: T

6th blank: R

7th blank: L

8th blank: T

The sequence of letters filled is: L E T A T R L T.