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

Let's count the total number of letters and blanks. The expression contains 20 elements. Let's look for a recurring word pattern.
Notice the segments K _ T G B and _ L T G _. This strongly suggests the repeating unit is a 5-letter block: K L T G B.

Let's test the block KLTGB across the entire length:

K[L]TGB→ KLTGB

[K]LTG[B]→ KLTGB

KL[T]GB→ KLTGB

KLT[G]B→ KLTGB

The missing letters sequential list is: L, K, B, T, G. This corresponds exactly to Option A.