Section 37 CPC defines the Court which passed a decree broadly for execution purposes, and that definition is the key to this question.
Reading clauses (a) and (b) of Section 37 together, rather than in isolation, shows that competence is shared rather than shifted, so the correct answer is in either of the court under a or b.
This question is best resolved by considering why the law preserves the original court's power even after territorial jurisdiction shifts, rather than moving strictly through the statutory text.
Balancing convenience and continuity in this way confirms that the correct answer is in either of the court under a or b.