Complete the series by inserting one letter per gap group (left-to-right). Lowercase only. Series: mc _ m _ a _ ca _ ca _ c _ mca

Introduction / Context:
The stem alternates fragments “mc” and “ca” with interleaved gaps and an “a” pivot, suggesting a cyclical cadence over the triad {m, c, a}. We must place six letters, one per gap group, to sustain that cadence and resolve the terminal “…mca”.

Given Data / Assumptions:

• Stem: “mc _ m _ a _ ca _ ca _ c _ mca”
• Gap groups: 6 (X1..X6), lowercase.
• Anchors: the string already contains “mc”, “a”, “ca”, “ca”, “c”, and closes with “mca”.

Concept / Approach:
We look for a minimal repeating insert that cycles through c→a→m in a way that completes “mc…a…ca…ca…c…mca”. The option “camcam” offers exactly such a c–a–m / c–a–m double cycle.

Step-by-Step Solution:
1) Insert “c” after initial “mc” to start a “mccm” bridge.2) Next, “a” to pivot toward the “a” anchor.3) Then “m” to rebalance toward “ca”.4) Repeat the cycle “c a m” across the remaining three gaps to align with “…ca … ca … c … mca”.

Verification / Alternative check:
The double-cycle “camcam” threads seamlessly, preserving the seen bigrams and landing on the terminal “mca”.

Why Other Options Are Wrong:

• acmmma: Produces long “mm” runs and misaligns the final “mca”.
• aaacmm: Overloads “a” and postpones necessary “c/m” placements.
• acmmc: Too short (5 letters) for 6 gaps; also misaligns cadence.

Common Pitfalls:
Assuming each underscore equals one letter of total, rather than one letter per gap group; ignoring terminal alignment requirements.

Final Answer:
camcam