Step 1: Picture the problem.
A decree was passed against several defendants. One of them never got the summons and so never had a chance to defend himself. The question asks what remedy this defendant can use under the Code of Civil Procedure, 1908.
Step 2: Spot the key fact.
The important point is that this defendant was not served with summons. A decree passed against a person who was not heard is called an ex-parte decree, meaning a one-sided decree.
Step 3: Find the right rule.
Order IX, Rule 13 of the Code gives the remedy for an ex-parte decree. It lets the affected defendant ask the same court that passed the decree to set it aside.
Step 4: Match the grounds.
Under this rule, the defendant must show that the summons was not duly served, or that he was prevented by a good reason from appearing. Here the summons was not served, so the ground fits perfectly.
Step 5: Rule out the other options.
Appeal (option 1) is not the only or best route here. Review before the same court (option 3) is for errors apparent on the record, not for non-service. A separate suit (option 4) is not the correct path either, because the Code already gives a direct remedy.
Step 6: State the answer.
The proper remedy is to apply to set aside the ex-parte decree. \[ \boxed{\text{By applying for setting aside the ex-parte decree.}} \]