Directions: Select the combination of letters that when sequentially placed in the gaps of the given letter series will complete the pattern.

a b _ a a b _ c c a a _ b b b c _ c

The correct option is c b a c.

Step 1: Analyze the Incomplete Series
The given letter series with missing gaps is:
a b _ a a b _ c c a a _ b b b c _ c

Step 2: Fill the Gaps using the Combination "c b a c"
Placing the letters c, b, a, and c sequentially into the four blank spaces yields:
a b c a a b b c c a a a b b b c c c

Step 3: Identify the Logical Pattern
Dividing the complete letter sequence into sections reveals an expanding pattern of the letters a, b, and c:
• 1st set (1 instance of each letter): a b c
• 2nd set (2 instances of each letter): a a b b c c
• 3rd set (3 instances of each letter): a a a b b b c c c

Joining the groups gives the full pattern:
a b c | a a b b c c | a a a b b b c c c

Therefore, the correct sequence of letters to complete the pattern is c b a c.