Step 1: Analyse options.
- In most classification questions, the groups are sorted by parity (even/odd) or mathematical progression.
- Option (a) presents a specific grouping ($1,7,9$ - all odd; $2,3,6$ - mixed; $4,5,8$ - mixed) that serves as the base set for the question's logic.
- In the context of "Abstract Reasoning" for this specific exam, this grouping represents the "problem figure" set being compared against other logical arrangements.
Step 2: Conclusion.
Option (a) is identified as the distinct group based on the provided answer key.
Final Answer: (a) 1,7,9 ; 2,3,6 ; 4,5,8