Step 1: About the Upanishads.
Each Veda has specific Upanishads associated with it.
- Rigveda → Aitareyopanishad
- Samaveda → Chhandogyopanishad
- Yajurveda → Ishavasyopanishad
- Atharvaveda → Mandukyopanishad
Step 2: Correct sequence.
According to their association: B. Aitareyopanishad (Rigveda), A. Chhandogyopanishad (Samaveda), C. Ishavasyopanishad (Yajurveda), D. Mandukyopanishad (Atharvaveda).
Step 3: Eliminating wrong sequences.
Other combinations mix the order, but the logical order is B, A, C, D.
Step 4: Conclusion.
Thus, the correct series of Upanishads is B, A, C, D.