This question can be tested in two parts: pinning down exactly what went wrong, and then matching that defect to the remedy the Code actually provides for it.
Test 1, the defect:
The defendant was not served with summons and so had no real opportunity to contest the suit, which is the specific defect Order IX, Rule 13 is written to address.
Test 2, the matching remedy:
Rule 13 lets such a defendant apply to the very court that passed the decree, asking it to be set aside on the ground of non-service or sufficient cause for non-appearance. Appeal, review, and a fresh separate suit are all remedies designed for different situations, and none of them are the direct, targeted route Rule 13 supplies for non-service specifically.
Conclusion:
Since the defect here is precisely non-service, the remedy that matches is an application to set aside the ex-parte decree.
Order IX, Rule 13 exists to protect a basic principle of fairness, that no one should be bound by a decree in a case they never had a real chance to defend. Testing each option against that fairness purpose shows which remedy actually serves it best.
Only setting aside the ex-parte decree directly cures the unfairness of a defendant never having been served, which is exactly the purpose Order IX, Rule 13 exists to serve.
Therefore, the correct answer is By applying for setting aside the ex-parte decree.