Select the group of letters that, when placed in the blanks of the given series in order, will complete the series.

Let’s correct it carefully.

Series:
a _ b c a _ _ c a _ b _ _ a b c

We test pattern logically instead of trial.

Write positions:

a 1 b c a 2 3 c a 4 b 5 6 a b c

Now observe structure:

We clearly see repeating block tendency:
a _ b c a _ _ c a _ b _ _ a b c

Try splitting into segments:

Segment 1: a _ b c
Segment 2: a _ _ c
Segment 3: a _ b _ _ a b c

Now look for repeating pattern.
If we assume repeating cycle a a b c:

Repeat:
a a b c a a b c a a b c a a b c

Now place into original:

a a b c a a b c a a b c a a b c

Match with given:

a a b c a a b c a a b a c a b c

So it does NOT perfectly fit.

Now test option-wise quickly:

bacaaa → does not align pattern structure

abacaa → breaks middle symmetry

aaacba → introduces invalid sequence changes

aabaca → gives consistent alternating pattern

Checking aabaca:

Fills:
a a b c a a a c a a b a c a b c

Now compare structure:

• Keeps repeating aab / aca / b c alignment
• Matches positional constraints of repeating cycle best

So correct completion is:

aabaca

Final answer: aabaca