Because the defendant here never received summons, the natural first question is whether the Code has a purpose-built remedy for exactly that defect, and Order IX Rule 13 supplies the answer.
Since Order IX Rule 13 was designed precisely for a defendant who never had a fair opportunity to contest because of defective service, applying under that rule to set aside the decree is the correct remedy, not the other more general or narrower options.
An appeal and an application under Order IX Rule 13 can look similar at first glance, since both challenge the correctness of a decree, but they are built for different situations, and comparing them directly shows why the rule 13 route fits here.
Comparing the appeal route against the Rule 13 route shows that only the latter is designed to correct a decree that was passed without the defendant ever being served, making it the appropriate remedy.
Therefore, the correct answer is by applying for setting aside the ex-parte decree.