In the given letter series, some of the letters are missing which are given in that order as one of the following alternatives. Choose the correct alternative.

_a_b_abaa_bab_abb

The correct option is:

babba

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Step-by-Step Solution:

1. Count the total number of letters and blanks:

The series has 15 positions in total: _ a _ b _ a b a a _ b a b _ a b b

2. Divide the series into equal, repeating patterns:

Since the total length is 15, we can test dividing it into blocks of 3 or 5 letters. Let's divide it into 5 blocks of 3 letters each:

_ a _ | b _ a | b a a | _ b a | b _ a | b b

Wait, let's recount the string carefully:

_ a _ b _ a b a a _ b a b _ a b b

Total characters = 17. Let's look for a standard 18-character or 16-character repeating pattern by grouping them into units of 6:

Let's check the pattern b a b a a b:

• Block 1: b a b b a a
• Block 2: b a a b b a
• Block 3: b b a b b (ends early)

Let's test option babba by filling it in blank by blank:

• 1st blank = b $\rightarrow$ b a ...
• 2nd blank = a $\rightarrow$ b a a b ...
• 3rd blank = b $\rightarrow$ b a a b b a ...
• 4th blank = b $\rightarrow$ b a a b b a b a a b b a ...
• 5th blank = a $\rightarrow$ b a a b b a b a a b b a b a a b b

3. Break down the completed series into groups of 6:

Now, let's divide this filled string into groups of 6 letters:

• Group 1: b a a b b a
• Group 2: b a a b b a
• Group 3: b a a b b (which is the exact start of the next repeating group)

The pattern b a a b b a repeats continuously. Thus, the missing letters are b, a, b, b, a.