Select the ordered 4-letter tuple to place in the four gaps so consonant/vowel alternations remain stable: cab — c — bdca — d — abd

Introduction / Context:
This pattern has four distinct blanks and provides options as ordered 4-letter tuples. The aim is to maintain smooth consonant/vowel alternations and keep the relative ordering around “cab…c…bdca…d…abd”.

Given Data / Assumptions:

• Anchors: “cab”, an internal “c”, the block “bdca”, a trailing “d”, and the close “abd”.
• We must fill four blanks in the given left-to-right order.
• Letters a, b, c, d appear throughout; a natural ascending order avoids collisions.

Concept / Approach:
Try the ordered tuple (a, b, c, d): it supplies each missing letter once, matching the alphabetic flow seen in “bdca/abd”, and prevents duplicate collisions at any seam.

Step-by-Step Solution:

Verification / Alternative check:
Other tuples repeat letters prematurely or place identical letters on both sides of a seam, degrading regularity.

Why Other Options Are Wrong:

• d, a, b, c: Mis-times “d” at the front seam.
• c, c, a, a: Duplicates immediately, creating clumps.
• b, c, d, a: Twists the final seam against “…abd”.
• None of these: Not needed since (a, b, c, d) fits.

Common Pitfalls:
Forgetting that four blanks are filled in order and testing only the first seam.

Final Answer:
a, b, c, d