In the letter pattern _cca_a_ca_ac_ab, which sequence of letters should fill the blanks so that the resulting string follows a clear repeating pattern?

Introduction / Context:
This question focuses on pattern completion within a string of letters containing blanks. Instead of a direct alphabet series, we are dealing with a repeated block pattern embedded in a longer sequence. Such questions check your ability to notice repetition and structural regularity rather than simple arithmetic progressions on letters. Recognising the smallest repeating unit is the key to solving such problems efficiently.

Given Data / Assumptions:
- Incomplete pattern: _cca_a_ca_ac_ab - There are five blanks to be filled by one of the candidate sequences. - Options provide possible five letter sequences to insert. - The final completed string should show a clean, regular repetition.

Concept / Approach:
The central idea is to test each candidate sequence of five letters by inserting it into the blanks one by one, respecting the order of blanks. After insertion, we read the resulting string and examine whether it follows a simple repetitive pattern. The best pattern is usually one in which a short segment is repeated several times without conflict. Instead of random trial and error, we look for clues in the existing non blank letters, such as the recurring fragment "cca" or "cab," and see how a candidate sequence can create complete repeated blocks around those fragments.

Step-by-Step Solution:
Step 1: Mark the structure of the pattern. Initial form: _ c c a _ a _ c a _ a c _ a b. There are five blanks in total. Step 2: Try option A: abcbc. Insert letters in order into each blank: Blank 1 → a, Blank 2 → b, Blank 3 → c, Blank 4 → b, Blank 5 → c. Resulting string: a c c a b a c c a b a c c a b. This can be grouped as: accab accab accab. The segment "accab" repeats three times without any conflict. Step 3: Check that "accab" is a consistent repeating unit. First block: a c c a b. Second block: a c c a b. Third block: a c c a b. This is a perfect repetition, showing a very clean pattern.

Verification / Alternative check:
If we briefly test other options, we obtain less regular structures. For instance, other choices either break the alignment of "cca" segments or create mismatched endings where the final few letters do not mirror the earlier pattern. Only option A produces a string that can be divided into identical blocks. The discovery of three identical segments accab, accab, accab strongly indicates that option A is the intended completion. Since exam patterns usually reward the neatest and simplest repetition, this is a strong confirmation.

Why Other Options Are Wrong:
B) abcbb creates a string where the segments are not equal and the pattern is disturbed in the middle. C) abbcc disrupts the neat "accab" repetition and leads to inconsistent groupings. D) abccb similarly fails to produce equal repeated blocks and introduces irregular transitions.

Common Pitfalls:
A common mistake is to focus only on matching local fragments like "cca" without checking whether the entire final string has a global pattern. Some learners might also check only the first five to seven letters and ignore the remainder, missing mismatches toward the end. A better strategy is to always read through the full constructed sequence and see whether you can partition it into identical blocks of letters. If you can, you likely have the correct option, especially in questions that clearly hint at repetition.

Final Answer:
The sequence of letters that correctly fills the blanks in _cca_a_ca_ac_ab to form a regular repeating pattern is abcbc, giving the final string accabaccabaccab.